We do have an integration with LearnDash Groups on our group features.

Based on LearnDash groups, you can set your groups to premium by setting first the group access option on LearnDash Groups. You can find check this guide on how to do so: 

https://www.learndash.com/support/docs/users-groups/groups/group-access-settings/

Once you have created a group from your LearnDash groups, you can either associate it with any of your existing groups. You can find this guide useful on how to apply it to your courses: https://www.buddyboss.com/resources/docs/integrations/learndash/courses-with-social-groups/

All of the group's content such as courses, forums, activities and etc.. will be restricted based on the restriction of each of your groups.