Can not add user when user group is not 1

[wangyue0727]

The admin user can not add other users when the usergroup is set to values other than 1.

Issue reproduce:

1. Get a new copy from git: git clone https://github.com/Peilin-Yang/virlab.git
2. Rename it as temp, for example: mv virlab/ temp
3. Overwrite the conf file: cp /var/www/html/demo/conf/conf.php temp/conf/conf.php
4. Edit the usergroup in the conf file, change it to a value other than 1: if (!defined('MyUserGroupID')) define('MyUserGroupID', '5');
5. Link the index: ln -s /var/www/html/UD17EXT/index /var/www/html/temp/index
6. Sync the default users:
   rsync -az /var/www/html/UDSummer17/users/0 /var/www/html/temp/users/
   rsync -az /var/www/html/UDSummer17/users/1 /var/www/html/temp/users/
7. Change the permission: chmod -R g+w /var/www/html/temp
8. Login the new instance as admin.
9. Add a user as the admin,
10. Click Manage users, The newly added user is not in the list. And the new user can not login the system neither.