I personally played as a melee fighter and there were one of two more of those. A lot of the time one of the mages would pull with a long distance damage spell and melee fighters would intercept the mob and get its attention - worked well most of the time. There is a taunt skill as well that should get aggro on you when you use it - I only had it at level 1 though, so it was not enough to attract the attention of the Scary+ mobs we were fighting.
Mages would then use damage spells, and very important, heal our health and stamina when we got low.
I know there are also a number of aura spells/skills in the game and perhaps other buffs (self or other) as well, so there might be a role for specific "buffer character" as well. Root and fear spells are also in the game, so crowd control may play a part at higher levels as well - especially since trying to pull "just one" does not always go as smooth as hoped
.I do not have very much experience with (high level) group dynamics from other games, but it looks to me like putting together a team with different roles is very beneficial in this game (just came from CoH where just about anything you could scrape together seemed to get the job done).
The skill system makes this a bit different than the traditional class based systems though - in a very positive way, because characters that have both high magic and combat skils may take on different roles depending on what they feel like, or need xp in, and what the group can aggree on.