Simple Press said
Open a ticket...add your credentials...make a video showing exactly what you're doing...
Working on that now, but I just figured out what's going on...
I'll need your team to figure out why, and develop a fix:
I can filter the list and add a user to the user group only if that user is not already a forum administrator.
Make that user and Admin, and he cannot be added. Remove forum administration capabilities, and the user can be added. Something is going on under the hood to prevent a user from being added to a group with capabilities similar that which the user has.
Please try to replicate while I get a screen capture together.