I've run across a few non-members on the claims here, and usually handle it the same way, let them poke around and invite them to a chapter meeting. I also give them my name in case another member comes along and gives them a hard time. It all depends what kind of an attitude they come back with. Will usually ask if they are members of the Association, rather than GPAA members. If I get a blank look or "what association" then I know they aren't. If you ask GPAA? the usual answer is yes. Will takes written notes on date, time, # of people, plate #'s. equipment, etc. for future use if needed.
The second time I find them there and they haven't taken me up on the offer, I'll ask them to leave and take plate#'s and pics.