well I'm paying PGC member, I'm logged in, my gc.com account name is known to PGC, so this is just a matter of comparison of my logged in account name with the name specified in the mapcompare ... or adding the VGPS (polygon) controls to the live map.
and btw. this should also work with my gc.com friends, as far as they also share their finds with their friends (me).
So as long as PGC has access to the information about the sharing details, friend status, API Access, it is matter of implementing those kind of cross checks and make this happen for those scenarios.