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.

[MiRai]

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

Reading this, I didn't understand... Why would someone already in your group, request to join the group?

Upon further investigation, it would seem that the invite command (in the right-click unitframe menu) changes to a "request to join group" command when the person targeted is already in a group. This must have been a fairly recent change, because, in the past, trying to "invite" someone who was already in a group would just spit out a message telling you that they were already in a group. In fact, it still does when you use /invite, and no pop-up request to join the group is sent to the other player, even if they're the leader.