You clearly are pretty smart 
!
Yes - that is integrated tightly / managed brilliantly by the subscriptions plugin.
While you have a lot of control of how much folk can interact with each category, I’m afraid that visibility is somewhat binary by design: either they see everything or they see nothing (not even that it exists).
However, this can be built upon via these customisations:
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using subcategories
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put visible stuff in main category (or a less restricted sub-category)
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put hidded stuff in a more restricted sub-category
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making use of Category Previews
- this theme-component reveals the existance of a private category and allows you to display a bespoke page instead to those locked out
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Advertise activity in a private category (discourse-category-lockdown)
- this plugin reveals just the Titles of topics in the category and allows the content to be searchable