Many Mikvahs these days have a gate that opens upon swiping a pre-paid card. I've had many times where a person will stand there and wait for me to swipe and go in with me after I swipe my card.
Do I have to worry about Geneiva and not allow him, or is it completely on him and it's the person getting in without paying who is Oiver on Geneiva. ?
This question applies not necessary on Mikvah gates, that's just a scenario where it happens a lot.