If I’m not wrong, looking at the admin panel, it seems this can be done with privileges as well, similarly to that mentioned topic, but for specific private groups and categories:
Admin > Manage > Groups > + > Create Group
Admin > Manage > Privileges > [category instead of global] > Add Group
And set access, etc. privileges for that group of users and private category only.
Then, to add users:
Admin > Manage > Groups > [group for that category] > Edit > Add User to Group
It's possible to also give user-specific privileges such as moderator in the Privileges section. Or probably better using:
Admin > Manage > Admins & Mods > [category] > add-moderator
However, even with moderators, the site admin would need to create the categories and groups, set privileges, etc.