Clerics I guess. Many level groups have 1-2 clerics (sometimes a friar), a matter cab and icers. Sorc/minstrel/servant nec for mana supply. That's your common pet pull group in alb. Anything else is mainly getting invited for the exp bonus and/or protection of the afore mentioned classes (e.g. peeling/blocking melees).
That's my experience so far but I didn't get that far at all yet so others might have gotten different impressions.
Well the game really starts at 50 so do you mean in rvr? Becaus in that case- totally different question. Clerics always but then need mins for speed, sorc for mez , cab for near sight - kind of the core for rvr.