There should be a CSS solution that would hide those groups from people not in the paid group (it might need to be their primary group). Someone clever could bypass the css, but it might be good enough for your purposes.
To really enforce it, you’d need a plugin.
But why do you need the groups? If what you want to do is keep people who are non-members from seeing topics in those categories, you could just have the pay group be what controls access to the categories.
We’re looking to create more Facebook-style groups (hate to compare it to that, sorry) where members of those groups effectively get their own sub-forum, chats, etc. So, folks will need to be able to request membership into specific groups in order to gain access to those areas. Full discussion on the topic below:
I understand there is core functionality coming on this at some point, and I’m hoping it is nice because if we can’t get this working in our community there is a good chance we’ll need to move to a different platform. The socialization/group aspect of our community is super important to our growth plans.
If you’re self-hosted, a plugin to allow access to groups only for those in a particular group shouldn’t be that difficult to create or commission. Maybe $1000-3000? Depending on features and specs that test that the plugin works as expected.