That doesn't negate OP's original point that they should make it possible to contact them. What if he had found an error on an obscure page that they might not have noticed, or a security hole instead?
security.txt file in the well-known?
Anybody really use that?
If you have meetup organizers paying you hundreds of dollars per year, the least you can do is have a support person (or a bot) fire out a canned email response to the effect of “we’re aware of the issue and working to fix it.”