If you're going warlocks, I'd probably go with druids for the healing partners.
Either 3 locks and 2 druids or the reverse.

Druids give you Lifebloom and Rejuvenation as instant casts.
Regrowth as a fast casting HoT.
They can spec'd a fair ways into Balance for fun that way.
Insect Swarm and Moonfire as two dots for offense.
They add root, battlerez, normal rez, mark/thorns etc.

Three locks gets you a lot more offense.
Affliction - Corruption, Agony, Haunt, Unstable Affliction, Immolate.
Destruction - Corruption, Agony, Immolate, Conflagrate, Chaos Bolt, AoE stun.
Demon - Corruption, Agony, Immolate, Felguard, Immolation Aura etc.
Plus 2x Insect Swarm, Moonfire, Rejuvenation, Lifebloom.

Alternatively say 1x Disc, 1x Resto (Dru), 3x Warlocks.
Priest has SW: Pain and Devouring Plague, to match the Moonfire and Insect Swarm.
You get PW: Shield and Renew as instants instead of Rejuvenation and Lifebloom as instants.
Mass Dispel, Fear Ward, Shadow Protection, Fort/Spirit to go with Mark/Thorns.

Two locks probably gets you Affliction x2 or Affliction + Destruction.
Even two destruction would get you...
3x Moonfire, 3x Insect Swarm, 2x Corruption, 2x Agony... which is still 10 instant cast dots on 2 casts.
With UA from one of the locks, no one will want to decurse them off.



Starbuck Jones in another thread had a click sequence posted where his druids were automatically casting some hots on everyone and extra hots on the tank, as part of their dps click spam macro's.