And by the way, why can't a user be member of multiple user groups? You might want to give the user access to different forums, depending upon the users various memberships or products purchased in the woocommerce store. Right now it is only possible to have a single user group for the user profile.

This is already discussed here:

https://wpforo.com/community/how-to-and-troubleshooting-2/users-in-multiple-groups-possible/