Monika it’s considered really bad etiquette to @ people, especially those not involved in the Topic. We are all busy, some more than others. Bear in mind people may get notified during unsociable hours. Use that feature responsibly
To your issue, there are some auto-generated groups that you might be able to use, e.g. trust_level_0 contains everyone with an account and is maintained by Discourse itself, so no need to individually add each person. Giving that rights and excluding ‘everyone’ will prevent anonymous users seeing that Category.
If you need something more tailored because you have a mix of members, some of whom you don’t want to permission for this Category, you may need to create a group with only firm staff. You will need to manually manage membership going forward, but once you’ve set it up it will be less work to maintain. You will have to remember to onboard people though as and when.