So I have managed to work out most of the answer to this question based on your support article https://theeventscalendar.com/knowledgebase/making-members-calendar/
Now I can hide a category.
I want to make this category viewable to those I have already provided access to. Logged in users who are in the group ‘staff’ based on ultimate member settings.
So my question now is how to edit the css for logged in user in group staff
body.logged-in .type-tribe_events.tribe-events-category-members-only {
display: inline-block;
}