WordPress.org

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.