Sorry! This invitation is intended for new users, who do not already have an existing account.
I have created dummy users so that I can test rights to make sure they work properly. The accounts are all from the same domain, but have different emails. The first one added fine. When I tried to add the second one, I got the message above.
I tried searching, but can’t find anything related which doesn’t mean it’s there… it might mean I don’t know how to look.