That must've been really fun for your team, although they could just go AFK I guess. You should try this with other bosses and report your findings - unfortunately I can't since i'm not a TR.
I did it in a group with a GWF, two wizards and a trickster with me as the cleric; the wizards focused on the boss while the melee guys stayed with me killing the adds. Similar to that red dragon fight at around 56 - the dragon bosses just does those red area attacks so you can focus on the adds that keep spawning while…
If you remove them from the group you can just request reinforcements and it'll queue someone into your group. I've never been the leader so I presume you just right click your portrait but i've had this feature work at least twice.