Is this just a private message? Neither the group owner or I (as moderator) can see a notification where we can just click approve/deny directly from the request.
Yes, when a user requests to join a group by clicking the group’s Request button, a PM is sent to the group’s owner. The owner of the group then needs to manually add the new user to the group.
I assume that works for TL0 users even when TL1 is required to send PMs?
Yes, if a TL0 user clicks the group’s Request button, a PM will be sent to the owner of the group.