All of my slave characters have assist keybound.
I check the arrow in keybinds for character specific bindings.

Basically my pally (account 3) leads that team.
My warlock (account 5) leads that team.
My disc priest team's leader is on (account 1).

Because keybinds are saved on a per character basis, one assist click (nearer mouse thumb button) has them all assist.

I've personally given every toon the same interact key.
The further mouse thumb button.
So for me, its left click to select the NPC, thumb click for the slaves to assist the main, and thumb click for everyone to interact.
If you were to give them different interact keys, then you'd need a click per keybind.
I generally see an advantage is everyone on one keybind, and possibly melee on one not shared by ranged/heals.
Click the one everyone is on, when you wish to interact with a quest giver.
Click the melee only one, when you want them to run to a mob and engage.