Fix adding to administrators group in Web UI

Steps to reproduce:

1. Log is as Administrator
2. go to Administration > Users & Groups
3. search a user
4. select this user
5. in the User screen, click on ADD TO GROUP button
6. type adm in the search field and select administrators

=> the operation fails with the message

Only Administrators can change Administrators group

whereas Admnistrator belongs to administrator group

See attached screenshot