Unable to manually add user to Group with email-address as username via Admin UI

[elangley]

From ​#BB2425:

Steps to duplicate the issue:

1) Create a user with an email address as the username
2) In the Admin Panel go to Groups/Edit/Add? New Members
3) Type in part of the username and click to select the suggestion. The name shows up under the field with an "x" next to it
4) Click the save changes button in the upper right hand corner
5) A message states that the “The group has been updated successfully” but the member is not added to the group.

[DJPaul]

Duplicate of #5187