Tank
Make sure you have marked the mobs so everyone in your group knows which one to attack, which one to crowd control, etc.
Use your AoE abilities to hold threat on secondary targets while your group kills the primary target. The Death's Head Seer is the most dangerous, so mark him with a skull.
Damage Dealers
If your group is using CC on the adds, ignore them and damage Roogug.
If your group is not crowd controlling the adds, focus them down first, starting with the Death's Head Seer.
Pay close attention to your threat, as your tank might struggle to hold aggro on 3 different mobs.
Healers
Wait a few moments before you cast your healing spells when the fight begins, or you may pull threat on the adds.
You will need to react quickly if one of the DPS players gets aggro on one of the mobs and starts taking damage, as they can die very quickly.