Drambuster.- your club's constitution would normally lay out terms and conditions for Members' guests...number at any time, number of visits etc. this stops one person bringing in a charabanc full of mates,
OTOH, in the case of a celebration, it's not unknown for random members to sign -in the permitted number of guests,until all are legal.
(you DO have a visitor's book??? )
Unless you specifically demand AOPA membership, I'd suggest a wider scope , to take in , say, LAA, BHGA, BMAA etc. members....remember this is only a "grockle" filter, you'd still get their landing-fee, or day-pass fee, if that's the way you went.
The Sailing-Club was affiliated to the RYA and therefore paid a goodly chunk of cash to them each year, so it could be argued that members had already paid for the reciprocal arrangements with other clubs.
We also had "invitation meets" which closely parallel the "fly-ins", Again, both visitors and members paid a small fee towards the races and the ensuing trophies and sustenance. all entrants had to produce a valid insurance certificate.
It's the details that take the hammering-out to try to eliminate inconsistencies and unfairness, both to the members who'se substantial annual fees keep the club afloat, balanced against the fellow enthusiast who wishes to "sound-out" the atmosphere and possibility of joining.
I don't think there isany members' organisation, Trade or Leisure, in this country, that doesn't struggle to recruit and retain members.
Recently, I've had Flyers from the local Golf club, Conservative Club and Health club, all extolling their virtues and offering low-cost entry.
5 years ago, they all had waiting-lists!
Now, Don't get me started on the difficulty of finding volunteers for committee and official duties!