Some aspects of this site are private to logged-in members and some technical pages are available to administrators only.
The visibility of page tabs across the top of the page are set in the menu item entries. Appearance>Menus
Making a page tab invisible does not prevent a user from accessing the page directly if the know or guess the URL. To prevent this, critical pages have been made private by setting that attribute on the page.
Note that making a page private also makes it invisible to the menu editing panel. So it may be necessary too make the page public while setting its menu entry an then making the page private afterwards.
Full members belong to the group Registered, which gives them permission to post and see the list of members. See the capability in the Group control panel. You could create other classes of membership if need be.