I think you're confused - you only get the mob that's hitting you as your target if you have nothing targetted currently - in a group situation the warlock will be targetting you directly or /assisting you which will have your primary target as their target - either way targeting them and /assisting is giving you the wrong mob unless you lost aggro on the mob you were attacking...in which case you'd see the mob's implied target become your group member. You'd have to tell them that when they get aggro they have to clear their target with escape, then shout "on me", but honestly it's just easier to play in third person and pay attention to when a mob turns around, select it with your mouse and get aggro back. Or you could just be lazy and pop sentry watch and let them die.
- Rorrak