Outsiders triggering popup by requesting to join the group.

[Andreauk]

I wasn't aware that other people could request to join your group—I don't think that's a thing (outside of the BNet friends thing).

On the other hand, if you try to invite someone into your group when it's full, including re-initiating ISBoxer's invite macro, you will trigger that message. I would imagine this can also happen if you list yourself as a group in the group finder tool with < 5 characters, and without converting to a raid group before hand.

You can right click any group members portrait frame and request to join group.