Nabble has the facility for a "Members" group. You need to add and remove members of this group manually via the screen at:
Options > Users > Manage Users & Groups
and control how that group's access differs from other users via the screen at:
Options > Users > Change permissions
There is no facility for collecting subscription charges from users or monitoring whether dues have been paid. You would need to do this externally of Nabble's service. Perhaps the easiest way is through a host site in which you embed your Nabble forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.