Most servers are pretty dead. There is however, usually, 1-2 guilds that recruit most active players on each server.
As for good class combos...
Tanking: Knight/Champion
Healing: Priest/Druid
Melee Dps: Warrior/Rogue or Warrior/Scout for a little more range
Range Dps: Scout/Rouge
Caster Dps: Mage/warlock or Warlock/Mage depending on what you want more focus on.