Monk is actually a very strong support class. Great auras, Exploding Palm is godly, Cyclone Strike/LTK for crowd control, Inner Sanctuary, Breath of Heaven and Unity passive skill. Monks and Wizard together are INSANE!
For my high torment "dream team":
Monk's first. With area damage, nothing comes close to the DPS increase from cyclone strike. It also greatly increases the damage dealt by exploding palm. Pulling a crowd around an elite can be a huge time saver. Inner sanctuary: forbidden palace or the crowd control immunity is very useful as well.
Wizard's second. ~50% group DPS via "damage taken" debuffs with the "sleet storm turret" build I linked a while ago.
Third slot: barb. Potential for a big group movement speed buff with the new legendary belt. War cry is a stronger defensive buff than the remaining possible monk mantras. Sprint:forced march for more move speed. Tons of ways to generate health globes, which when combined with the reaver's wraps means HotA:birthright sustains itself. The item drop shout might be worth looking at too.
For the last one, I'm not so sure.
A second barb could work pretty well.
WD: potential for the best DPS in the game with the torment DoT set. Good uptime on the group-friendly cooldowns with the right legendaries.
Second monk: FoT:Static Charge is great in full groups, but in situations where it would be most useful they would already be getting clobbered by other area damage. Exploding palm boom stacks, but it's overkill since area damage is so strong with it that one monk is usually sufficient, especially with the ~100% palm Fist of Az'Turrasq. Inna's set gives the 4 base mantras, so additional monks just add runes and active effects.
Second wizard: could add a stretch time bubble, but otherwise doesn't stack.
DH: Marked for Death / Grim Reaper. Wolf pet's a strong cooldown. Seems like DH would be much better off in an all-DH specialist group where they can run at full speed.
Crusader: thinking this might be a solid choice when able to use the reaver's wraps to benefit from the barb's globes. Block-dependent effects unfortunately scale down in groups since attacks are distributed between targets. Condemn(?) for brief ~100% crit chance. Can help keep Nephalem Glory maxed out, although there are legendaries the other classes can use for that (e.g. Blind Faith + the one that gives NG globes on blind). Need to finish reading up on crusader legendaries and abilities.