In a group situation you're one of three things: the tank, the leader, or damage dealing. The tank takes the hits. To be an effective tank see Advanced Character Types. If you're a damge dealer, you are of one of two pursuasions: spells or hitting. Both are detailed in Advanced Character Types as well. The most important member of the group, however, is the leader.
All group members should know the area they are in, but the leader of a group should know the area especially well, and be able to instruct your party about special dangers in the area. For example, make sure your party doesn't blunder into the Royal Elite Guards while in Atlantis. A leader should also know everything there is to know about the mobs that the group is facing. This includes knowing what spells to use on them, what charisma they are aggro too, whether they will cast spells on all group members or just the tank, and he should have a general feel of when the mob is going to die.
The leader of a group should NOT be the tank. This allows him to assess the whole party's welfare as opposed to just his. The Function Mage, detailed in Advanced Character Types, makes a good leader.
A leader should know his group members strengths, weaknesses, and abilities. This would allow him better to decide when the tank is ready, what spells the mages should cast, and how much damage to expect out of a hitter. He should be good at giving orders, and instruct all the group members of their role to minimize fatalities. A responsible group leader cares about his group, and will do anything for them. Including sacrificing himself to save them. For example, taking hits while yelling "FLEE FLEE!!!" so that the weaker mages and hitters have time to escape.
It takes knowledge and wisdom to make a good group leader, most of us don't have enough of that. The few who do though, should take charge of their groups, and make sure they run okay. Half the reason high level equipment is so hard to get is because groups aren't organized. Have you ever heard the proverb "A chain is only as strong as its weakest link"? Well how strong would a pile of independent links be? I don't think it'll be pulling much weight.