If your only experience of grouping has been fighting Hogger, you probably don't yet know much about fighting in a five-man group.
There are four roles, normally: tank, DPS, healer and leader. Whoever is leader is also one of tank, DPS and healer. The group leader should normally be the highest level member of the group, or the most experienced member. If you are nominally leader, but not the highest level or most experienced member, you should invite the veteran to be leader. If you don't you risk the group having two leaders or none!
It is the job of the group leader to make sure everybody knows what is expected of them (Tank, DPS or Healer). Team members should follow the team leader's directions.
- Leader: the leader's role is to assign combat targets, and make sure that the team is ready before giving the attack order.
- Tank: the tank's role is to draw and keep the enemy's aggression. He is not expected to dish out punishment to the enemy, but rather to take it. Normally, this means defensive stance, shield and lots of taunts and mocking blows to keep the enemy attacking the tank and not the DPS or healers
- DPS: DPS stands for "Damage per Second". The DPS's job is to do maximum damage to the enemy in the shortest possible time, while keeping their threat to a minimum, thus making the tanks job easier.
- Healer: mainly the healer must make sure the tank, who is taking all the beating, is not killed. Of course, sometimes, enemies will attack DPS or healers, so the healers must also keep an eye on the whole team!
Before each fight, make sure you are buffed and have buffed all your teammates! Buffs are beneficial spells you can cast on yourself or your teammates, such as "Arcane Intellect", or "Mark of the Wild".
The group leader may assign targets to particular members. Otherwise, the usual target is whoever is beating on the Tank! Tanks should be in defensive stance, and should be aware that spells may attract mobs to squishy DPS spellcasters, and should make sure they maintain threat (for instance by taunting the mobs who are attacking DPS).
Don't pull prematurely. Wait until the leader gives the order before attacking. I would go so far as to say that the most common cause of wipes is attacking a target before the team is ready.
Don't run! If you're being beaten up and likely to die, call for help, but don't run. Running makes it harder for your healers to target you, and harder for your DPS and tank to target the enemy who is attacking you; and since the enemy can usually run as fast as you, you normally won't outrun them, anyway (and in a dungeon instance, they will never stop chasing you). Even worse, by running you may draw other enemies into the fight, and instead of just you dying, the whole team may be wiped. Even if you are definitely going to die, stand your ground. Better to take one for the team than to wipe the team.
I think it's best to stick to group loot. This gives everybody a chance to roll on valuable items, and because it isn't first-come first-served (like free-for-all) it doesn't encourage looting mid-fight. You can concentrate on the fight without worrying that a less-principled team member is busy robbing the bodies.
Normally, when you are given the chance to roll on a valuable item, it is polite to choose "Greed" or pass. If you really need an item, ask the group leader (in party-chat, not whisper) if you may roll need. Normally you should only roll need on items that you can use immediately (either you can equip yourself with the item, or learn to use the item). You should not roll need on an item because you have an alt that needs it, or because you want to disenchant it.
Don't loot while the fight is still going on. Sometimes it isn't obvious that a teammate is still in trouble because they're behind you. It would be a Bad Thing if you were looting while your teammate was being killed!