Sorry for giving the wrong information.

In wpForo, 

Only those users who have Admin Usergroup and Admin User Role can manage other User's Primary Usergroup from the front end. Also, the same Admin user can manage the User Secondary Usergroup from front End only if User Custom Fields Addon is activated on the website.

Please resend the website URL and the credentials with email.

It still does not seem to be possible because of the following two reasons:
- The wpForo Default Admin usergroup still cannot manage Users Secondary Usergroup UNLESS it has WP Admin userrole (but in that case he has access to the whole WP Dashboard, and he can delete other WP Admins).
- In the Front-End there is no usergroup shown for a member, neither a secondary usergroup.

Hope you have a solution (you can use the credentials of the second test site that I sent you, which has a clean installation, to see what I mean).

For now the only workaround is to install a custom WP Admin plugin to block access to WP Dashboard and other WP Admins and to add code to functions.php to remove menu items from the Admin Toolbar...

Only wpForo Default Admin usergroup can manage the Users Secondary Usergroups from the Front-End.

Primary usergroup from the front end can be changed only by default admin, other users can only change from the dashboard. 

Checking that part. Will let you know.