It's extremely rare (see above), but when it does happen we do everything we can to make it up to the other group (free drinks, free Groupers, profuse apologies, etc.) and lay down the law on the other group (including removing them from the community).

Have you sought statistics for no-shows that aren't reported. There's a general prohibition against reporting other people's failings particularly when trying to appear warm and accommodating.

Not sure how you'd do it except perhaps as an exit question for people leaving your community.