As Jay mentioned you can do the s via groups.
Create a Category you want visible but restricted.
Category Security Settings
- Everyone - See
- Group name - See/Create/reply
Now everyone can see the category but must join a group to create and respond to topics. Depending on Category purpose you could restrict who all can Create topics by adding a new group will full access with removing the Create permission.
You will want to edit this in Category settings.
To participate in category you need to <a href="/g/Pimaxians/members">Apply</a>
The above is what I use for a restricted category that requires joining a group to post.