How do we re-invite someone who left a group private message?

[Miles_ONeal]

Someone left a group message thread and now wants to return. Both a mod and I (who started the message) have sent an invite, but the member in question hasn't seen one in their inbox.

1. Is this supposed to work? (Those who left are listed as having "Permanently left".)
2. If so, where would this person be getting notified, if not their inbox?
3. If standard invites won't work here, what will?

Thanks!