Wizards seem a hard sell in groups many times, but a good wizard...oh who am I kidding...wizards suck!!
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Assuming you are ok with non-stop chain pulling, a monk is pretty nice for grouping. Monks also duo/trio well which is often better xp's. A well-geared monk can also tank pretty well while still pumping out good DPS and soloing is another option.
For group friendly DPS, it is hard to beat a rogue. I've leveled a few rogues on p99 and still find them fun to play, but keep in mind that rogues lack versatility. Yes, they can and do pull, but bards, monks, monks, and rangers are generally preferred. They are kings of CR's, but let's face it...groups don't generally get built around CR capability. What they do exceedingly well is crap out DPS and mana ain't a limiting factor for them.
If you want versatility, I'd say go necro since they can heal, give mana, CC, and eventually DMF which is useful in some areas.
I feel some people underestimate mage overall contribution (pets do a lot of damage, damage shield, + nukes), but I found getting groups on my mage is fairly easy.
And let us not forget the most powerful DPS which would be of course an enchanter. Their contribution to group DPS from brain buffs and haste makes them worth adding, but with a hasted pet their overall contribution to DPS is pretty insane. You will die more than a rogue though.