Select group on adding new user

Do you mean adding the user to groups when you create the user?

I looked at providing the possibility to specify the groups when you add a user directly on the back end, but last time I checked, it wasn’t feasible to add it due to a lack of hooks for the user creation admin page.

Or do you mean the groups simply don’t show up on the Users admin screen? Then that sounds like a conflict. Try disabling other plugins until leaving the essentials (Groups in this case), you’ll be able to choose the groups and add the selected users on top of the user table. Then enable additional plugins individually until that breaks and we have our conflicting plugin.