Backend: Can't add member to group if user already invited but ignores it

Hello.
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.

Thanks.