Admin controlled wordpress custom user attributes

I’d like to be able to have Admin-only checkboxes in user profiles, then access these attributes in php.

The usage scenario is giving certain users access to parts of the site. This is different to creating new roles and capabilities in that certain specific users may have more than one capability to view parts of the site eg.
Mary can access page A and B
John can access page A and C

I would probably implement this based on body classes, or something else, but the php-accessible Admin-only checkboxes in user profiles is the main functionality I’m enquiring about. I’m open to using plugins or code in functions.php.