Add user as group owner

Allow group owner to add existing user to the group. 
see https://trac.openmicroscopy.org.uk/ome/ticket/11908

To test:
 * Log in as user-1
 * Go to the Admin tab
 * Right-click on the group "private-1"
   * The "New User" option should be greyed out
   * The "Add existing user" should be enabled. Select and add a new user.