Welcome to the official discourse community!
What you are trying to do is allowing a specific group to see/create/reply in a specific category.
Fortunately for you, there are many options for groups. When you create a group, you can go to the settings and edit it to be public, private, or join by request. If you select the third option, a message will be sent to the owner(s) of the group and they will choose whether to allow the user to join the group or reject the application. If you want to have a group, where anyone can join, you just select the second option.