How to assign staff in Support System to categories

I've searched and can't seem to figure out how to solve this simple problem. I have staff I want to setup to handle support tickets in a category. But the person is not in the list of users to select from.

Does the user have to be a super admin or how do I set them up to be able to assign a category.

I also can't figure out how to assign a super admin as in the user profile the checkbox won't allow me to save.

Thanks