(these rules are very generic, and vary depending on the makeup of the group)
Although you sometimes have little control over whom you group with, you should try to make up a balanced group. A group of 3 wizards is all attack and no defense, and will probably lead to 3 dead bodies. A good group includes at least one of each of the three class categories I listed above. Of course many classes, especially Bards and Druids, can handle a little of each role. A tank is an absolute necessity, but in a pinch the pet of a necro or magician can sometimes work. An attacking magic user such as a wizard is also important to kill the runners before they go get help or slam them when the fighter starts to hurt. Never turn down the addition of a cleric and his healing powers or a enchanter and his buffing powers. When forming a group, remember that the experience and gold are split between the members. Thus, if experience and gold are the only considerations, a good rule of thumb is to form only as big a group as is necessary to beat the monsters you plan on attacking. Of course part of the fun of grouping is meeting new people and hanging out, so don't turn down an additional body just to squeeze a little more experience out of the game.
Before you start to hunt, establish the rules with your group. The group leader has a lot of responsibility for this. Leadership and party discipline can make or break a group. Determine in what order you will loot bodies and whether you will keep on autosplit. I would suggest you loot alphabetically and always split loot (just remember to turn it off before you get your corpse). It is the leader's responsibility to keep track of whose turn it is to loot. Make sure everyone knows their roles so that there is no confusion during the battle. A little talk before going out may save someone's life.
A basic rule for parties is that you want to spend as little time sitting on your butt as possible. If you end a battle with one of the fighters way down in hit points or one of the magic users way down in mana, the whole party will have to go sit and wait for them to regenerate. Ideally, everyone should be down about the same amount so that the rest is needed equally by everyone. A fighter who stands in and takes all the damage while the rest of the group stays healthy or a wizard who blasts indiscriminately and wastes all his mana on a weak monster will soon find themselves having difficulty finding a group willing to take them. Remember that mana regenerates much faster than hit points, but hit points can be healed by a cleric or bard.
Before you go out and start the hunt, designate who will take the lead. If you have a ranger, he should always take the lead since he can track the best monsters. Otherwise, it should be the group's tank. There are two basic strategies with this. Everyone can just follow the leader until he reaches an attackable creature then try and quickly form their places and attack, or the group can park in a defensible spot and send one member out as a runner to attack a creature and lead it back to where the rest of the party is waiting in ambush. Personally I prefer the latter strategy, but it does mean more time sitting around waiting for something to spawn. One point on this is that you need to follow the leader. This can't be emphasized enough. Most people play in first person mode and can't see if you are behind them. They have to assume you are following. If you wander off, you won't be there to help the others and they won't be able to come to your rescue when you get attacked by that mad grizzly bear.
Fighters: Your role is to make sure that the monsters always attack you and stay off of the spell casters. Don't forget your taunt skill. Use it to keep the monsters attacking you. You should go in first and soften the monsters up before the spell casters administer the killing blow. Since the fighter is the least easily killed, when the fighter yells run, the whole party should immediately hightail it to safety.
Magic Support. You should keep the whole party, especially the fighters, buffed at all times. If you think enough time has passed that your buffs may have worn off, remind the stupid fighters to let you know when they need more. Clerics should constantly watch the life meters of other party members and cast heal if they get below ½. If you have a decent attack ability, when you aren't healing or supporting, step in and take a few whacks now and then. Just make sure you don't do so much damage that the monster turns on you instead of the fighter. Also, remember that you can do more damage to the rear of a monster. You should also keep and eye out for incoming monsters. As soon as you see another approaching, yell out to your group.
Attack Magic. Stand back patiently while the battle begins. If you want to, go ahead and carefully step in and take a few whacks at the monster. Keep and eye on the life meters of the fighters and if they are getting worse than they give, help out by blasting the monster a few times. You don't want to attack too soon, since monsters have an annoying habit of going after the person doing the most damage, and you don't want it to focus on you. While it can be amusing to see a magic user running around in circles being chased by a monster while the rest of the party tries to hack at it, it is also self defeating (and usually bad for the health of the magic user). As soon as the monster takes off, cast root to stop it, or if it is really low on hit points just blast it down. You should also keep and eye out for incoming monsters. As soon as you see another approaching, yell out to your group
Except for wizards and fighters, most classes can cross over to some extent to these three main areas. Assign out the main roles, but don't be afraid to vary if the fight is going badly.
When someone yells run, don't hesitate to run to safety (hopefully you have already made note of where the nearest safe spot is). You know at least one of your party is no longer fighting with you, and your chances of not dying have dropped considerably. Besides, if you're playing with a good group, there's probably a good reason someone yelled run. Get out of there now.
Class Specific group strategies:
Warrior. This is easy. Attack. Attack. Attack. Also, protect your spell casters. You should be the one who runs out and initiates the combat whenever possible. That way you are the one the monster focuses on from the start. Use your taunt whenever the monster decides to go after one of those weak mage types. Don't forget to use your combat skills. If your hit points are getting low, back away and let someone else take some damage, even a spell caster. Dying doesn't help your group any. Fighters of all types are good group leaders since they are the ones who initiate combat most of the time.
Paladin. Same as warrior for the most part, but you also have the ability to buff ahead of time to make the party stronger and heal al little after the battle. Save your lay hands for the most desperate situation. If someone in your group is about to go down, use lay hands. Better to lose it in a day than waste it.
Ranger. Same as warrior as far as the tank abilities. In addition, because of your tracking skill, you will probably be the leader when outdoors. Track a monster that the group can handle and lead it back to the group to be disposed of. Or if you prefer, track the monster and lead the group to it. If you are doing this, you should change your views so that you can see behind you and can be sure that everyone is actually following you. Also, keep and eye out for killer monsters and warn your group if they are in the area.
Monk. Same as warrior in most respects. Use your feign death skill. If the group is in trouble, you should be the one to buy time for the rest of the party since you can use feign death to convince the monsters you are dead and maybe help the entire party survive.
Shadow Knight. For the most part, you are a dark warrior. However, do not forget the necromancer in you. Blast with a dot spell first then attack.. attack.. attack.. At a lower level (1 - 10), you have no business grouping, unless you just want to socialize. Your first dot spell comes at level 14. Treat your pet as a dot, not a tank. He will always be half your level. Keep the mob away from the casters and healers and use your harm touch whenever you feel necessary. Remember harm touch work best when mob is not attacking you. Just like Paladins use their Lay Hand to heal a group member to full when in trouble, use your Harm Touch to kill a mob who is about to kill a group member, not just for yourself. Harm Touch will come back in 24 eq hours, but the friends you meet may not.
Rogue. Unless you don't have a tank, you don't want to be the focus of the monster's attention. Instead let the fighter type get its attention and get behind it and backstab. Keep backstabbing. This is a great skill for grouping. Also, if you have the time, you might as well pick the monster's pocket for some extra cash. If you are in a situation where nobody in the party knows what's ahead, use your sneak skill to scout the area.
Cleric. You are not a tank. Everyone is relying on you to keep them alive. This is a big responsibility. Watch your party members' life meters and heal anyone who starts to get below half. When healing is not needed either step in and take a few whacks or stay back and keep and eye out for any new creatures so that you can warn your party. Prior to the battle, you should buff up the party to keep them strong. Also, don't forget lull if you are planning on attacking a group of monsters. Don't waste your mana. Your heal spells may be the only thing standing between the party and disaster. The ideal strategy for a group with 2 tanks and a cleric is for the tanks to alternate attacks, with one attacking and the other standing back getting healed. This way you can keep a full strength fighter on the monster at all times.
Shaman. Your strategy varies a lot depending upon the makeup of the group you are in. First off, cast SOW on every group member. It is the best spell in the game and will save your party's lives many times. If you have a lot of firepower up front, stand back and heal when needed (like a cleric), balst a little and make sure your fighters are buffed and ready to fight. If they get in trouble, wade in and start bashing or cast damage spells. You can dish out some decent damage.
Druid. The druid's strategy really depends upon the makeup of the group. One thing you should always do is cast SOW on every group member. That alone gives your party a beter chance of surviving an encounter. You are probably the travel guide for the group because of your travel spells. Prior to the battle, keep the fighters buffed. Once a battle starts, cast your damage shields on the fighters so that they can start hurting the monsters back when they get attacked. Then your actions depend on your role. If you are the parties only healer, try to save your mana for heal spells and play it much like a cleric. If you are filling in on the wizard role, then by all means strike down the runners. Ideally, given time to meditate, you can buff the party before combat, destroy creatures during it, and heal everyone afterwards. (Thanks to Starkinder for the input on Druids)
Bard. While not the greatest tank, you can certainly play that role. But you are more than that. Like a cleric, you should always watch your party's life meters. If everyone is high, either play a buff song, battle song or damage song. It really depends on what you're attacking and how you are doing. As soon as people start to drop, play the heal song. You are crucial to the groups survival, since as long as you are alive the group can outrun any monster. If the party looks like it is in trouble, switch to the accelerando and tell the party which way they should run. If you can keep the party together, you should be able to help them survive by running away faster than the monster can pursue.
Magician. Let your pet do all the work for you. Don't even look for a group unless you have plenty of Malachite to summon pets. A magician without a pet is really just a weak wizard, and not very much help to a group. Remember that his death means nothing, and a group members means everything. Send him into the hardest places. Support him with spells, but don't let your mana lag too far behind. How you do this depends upon whether you are the only spell damage dealer in the group. You are also the group's provisioner, especially at low levels. Volunteer to stock up your group members with food and water. It doesn't cost you anything other than mana. This will make you very popular, since at lower levels those items cost enough money to get really annoying, and it's a real pain to have to go back to town just to buy more bread. Many people don't realize you can summon food and won't ask, so be sure to volunteer it to the group.
Necromancer. Let the skeleton do the nasty work. Since your spells tend to do damage over time, if you plan on casting them, cast them early. It might be a good idea to cast and then immediately sit and meditate. You can never have enough mana.The necro pet is usually 1 level below the level of the necro. At 20th and up they have magic attacks, dual wield and riposte. The pets can be buffed and with the bareknuckle delay are a very good addition to a group. Necros are particularly good group members if you are fighting a large crit. Their damage over time spells at 20 will reach over 800 when combined and at 29th well over 1000. Combine this with the pet damage and the 95 + DD spell against undead and 65+ DD against live creatures and you have a damage dealing partner. Necros are also good paired. They work very well with a druid. One warning sent to me: When grouped with 4 or less the group needs to treat the pet as a player in terms of making sure damage is shared. The reason for this is (unless you are fighting blues or yellows in which case why be grouped ) the damage over time spells will direct the creature to the necro or another party member like the wiz if the pet dies. I have seen many times in smaller groups...pet dies group flees and dies. Now this is not to say that the pet should not die first if necessary, it definitely should, BUT the crit being attacked should at least be at a level of health that the group can finish off with a surety.
Enchanter. Keep your party buffed at all times. Before the battle starts debuff the tougher monsters to make them easier to kill. One of the better spells of the enchanter is lull. If you are thinking of attacking a group of monsters, but not sure you can take the entire group, lull the ones you want to stay back and pick them off one at a time. With an enchanter, a group can take on some of the camps and survive. Then you need to assess the balance of your group and whether you need to blast or let your pet do the work. If the battle is going well, there is no reason not to step in and attack with your dagger or staff.
Wizard. Conserve your mana. Let the fighters start a battle and sit back and watch. You don't want to attack too soon and have the monster focus on you instead of the tank. Part of the way into the battle, you can go in and start to hit the monster for the little bit of damage you can do. It won't faze the monster so you are safe. If the fighters start to fade a little, blast the monster with a few spells. If it gets low, you can finish it off. If the battle starts to get desperate, blast it with all you have. You are the last resort in that case. If it starts to run cast root to stop it in its tracks or just blast it down. Always keep an eye out for incoming monsters so that you can warn your fighters. Remember you can regenerate mana about 3 times faster than the fighters can regen hit points, so once the party sits you want to have times it so that you need as much sitting time as the fighters.
Rule number one of soloing is to not do it (at least after the first 10 or so levels).
If you want to solo, this is my suggestion. Find a place where there is either a zone barrier or a guard tower and a where you can find a number of monsters who con blue. Keep within easy running distance of safety and look for monsters you can handle. Like a deer hunter, you may find a lot of your time spent sitting around waiting for your prey. Change your view so that you can see if anything is sneaking up behind you and attack anything that cons blue (you'll quickly learn what you can beat). Unless you are completely absolutely convinced you are one or two strikes away from killing it, if your meter gets below 1/2, turn and run to safety. If anything other than the monster you are attacking starts your way, turn and run for safety. If your mana gets below 1/3, turn and run for safety. Learn to be a coward and you may actually live a while. Keep in mind that if your place of safety is a guard tower, you may get hit once or twice before the guards manage to kill the intruder, so you should run even sooner. Those who solo spend a lot of their time sitting and healing or meditating.