I'll be honest, the only groups I tend to find on my wizard are ones where someone invites me to port them somewhere. No xp groups!
Then again, I suppose exclusively porting other people to their xp groups is kind of a specific group role
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In the rare occasions where you do find a group, being a wizard is easy as people have very low expectations. Drop a nuke when a mob is around 50% hp and you're set.
That said, wizards can be really useful in groups (rooting the tanks target to help him keep aggro, interrupting NPC casts with stun, using longer duration stuns for all sorts of things, casting snares, helping offpark adds with roots), but people don't expect it. They just appreciate it.