Last weekend, I tried the new feature of sharing lists, but as my test person (later on tour) told me, it can only be used, if one is premium member of pgc.
In detail he told be, that he got a notification in the pgc postbox about "being added" to one of my lists, but couldn't access it, because of being only basic user of PGC. I was really surprised and think, that this is a bug.
IMHO I think, its ok that the sharing initiator of a list might be in need to be PGC premium (that's me :-) ),
but I really expect to decide freely to whom to share a list.