First, some warnings:
My information may not be accurate and should be tested before being trusted.
I don't understand the thing about taunt and aggro skills.
Your mud may treat tanks differently than my stock mud does.
I think the things you need to remember here are :
1. The last person into the room is the first on the list.
2. Any entry trigger should wait at least 1 sec or the action may happen before the target enters the room.
3. Followers enter a room so quickly that we don't see a pause but the game itself does.
Therefore:
The mob needs to wait the usual 1 or 2 sec. By this time all followers should be in the same room as the mob. I'm not responsible for anyone who immediately leaves the room.
Do loop. For some reason I usually suggest the decapitating rabbit (tstat 1398) but the
grenade trigger of the day
will work with the benefit of being able to see it in the forums.
Check to make sure it's a player and not some mob that wandered in or a player pet that happens to be following.
(help char-var: is_pc - Checks if the actor is a player.)
Check to make sure that this character is a follower.
(help char-var: master - The master of the actor (as an id), or an empty return.)
You might have to play with this a bit but I think if !master the player is not in a group. In TBA it seems that it's possible to be in a group and not following but that doesn't seem to be what you're looking for.