Designate categories as "members-only" via 'Privacy Settings.' Assign them to users via 'Profile.'
If you'd like to make the content on your site visible only to certain users using categories, look no further!
Designate certain categories as "members-only" via Privacy Settings.
An administrator may then assign a user to them via Profile.
Only content belonging to "public" categories (any top-level category you do not designate as "members-only"), and categories assigned to the logged-in user, and all subcategories of either, will be visible.
I would like to thank the action hook pre_get_posts, without which this plugin would not have been possible.