@tkevitch I am new here but not new in this. I just want to say that To make a category visible only to specific users after they register, you can use Discourse’s Group functionality.
First, create a new Group and name it whatever you want. Then, add the users you want to give access to this private category to this Group.
Next, create a new Category and set the “Security” option to “Group”. Select the Group you just created as the allowed Group, and set the permission level to “Watching”. This will make the category visible only to the members of the Group and they will automatically be set to “Watching” the category.
Regarding your second question, if a Category has a privacy rule, it only applies to the Category and not to the Topics within it. Therefore, you will need to set the privacy rules for each Topic individually if you want them to be private to the same set of users. Alternatively, you could create Subcategories within the main Category and apply the same privacy rule to all of them. This would make all Topics within those Subcategories private to the same set of users.