removeUser(shortName)
This method removes a
    specified child user from this group. It does not delete the user.Parameters
- shortName
- The short name of the user to remove
Example
    var shortName = "MY_SUB_GROUP";
    var group = groups.getGroup(shortName);
    // now remove child user from this group
    
    var shortName = "james.joey";
    group.removeUser(shortName);        
        
    