I just got that message when i clicked on the Invite People button on a group i admin (Quick Moves).

Although I'm admin i have never invited anyone to the group before. So why does that message come up and not let me invite people.

Those could have been used up by either the super admin or one of the other admin

Ah i see, I assumed that there was a limit for each individual admin. So there's a limit for the whole group then.

Yes, its 30 per day per group.