Well, if you were a warrior, your* group would have a much better tank, and you'd be keeping all your friends, and their pets from harm.
If you were a priest, the Shaman wouldn't have to focus on healing at all, and then the Warlock, the Hunter, and the Shaman could go all out and try to kill their oppenents as fast as possible, and you could sit back and keep them alive. (But I've heard Priest isn't just sitting back and keeping people alive, but I've never gotten it past level like 7, so I don't really know, lol.)
Or, you could be a Rogue, and just tear through things as fast as you can alongside your friends.
But hey, I've posted how three very different types of classes could work with your group, a tanking class, a support class, and a damage class. So you can really be whatever you want, and be just as good in a group as anything else, WoW is just set up like that. :) You'll get used to it once you start playing.
Edit: Bad Grammar.*
Edited, Thu Dec 16 08:10:14 2004 by GrandMasterDaveo