Convoro has one idea for visibility and uses it everywhere: things are restricted to groups.
Forums, blogs, calendars, photo albums, Pages, Knowledge Base articles, menu items and widgets all have the same control — a list of groups with tick boxes. Tick nothing and everybody can see it. Tick one or more and only members of those groups can.
🚨 A signed-out visitor is in the Guests group, not in no group. That is what lets "visible to everybody" and "visible to signed-in people" both be expressed with the same control: leave it empty for the first, and tick everything except Guests for the second.
Administrator and moderator powers are flags on a group rather than per-person settings, so you grant them by putting somebody in the right group.