Not sure what you mean - are you trying to stop someone seeing a list of members?
As I said, the membership list of a group can be hidden (to only admins, or +staff, or +members, or everyone), but if you are creating a category where any member can participate (Tutorials), then their details will show when they post any content - so what are you trying to achieve?
EDIT: saw your reference to ‘read only’ afterwards. In that case it still works as they are not posting. You cannot see a list of members who have access to a category.
However, word of warning, by default, the site will show a list of all ‘Users’ in the hamburger menu and so you will also need to hide this in the menu using CSS