1) Invite user1 to private (invite only) group
2) user1 ignores it or reject (I don't know exactly)
3) try to add this user1 to group in backend: Groups > GroupTitle > Members > Add members (button), use "serach"
Result: You can notice that it's impossible to find this user1.
I suppose it happened because this user already invited before.
Expected result: Admin still could add this user1 to group manually in backend.