Restrict a category to certain readers

To restrict a category or sub-category to certain readers, create a reader group and restrict the category to the reader group. When a category or sub-category is restrict to a reader group, only readers who are part of the group will be able to access the category. 

Reader groups restricts waterfall, meaning that if you apply a reader group to a category, the same permissions will also apply to any content inside the category. If you want to restrict everything inside a category, you only need to set the reader group restrictions on the category as the restriction will automatically be applied to the content inside the category.