When you delete a member group with users in there, the users (name) in the search member option is gone. They don't have an assigned group so they can't log in.

I found a way to ban the member to get the name to show up then change them to registered user. Then they are able to log back in, but after a while the system remove them out of the registered user group and the name is removed again.

Its a bug in the system.

did u unban them after that ?, u should left the ban after u change their usersgroup