Or my recent thread…
The ending of my story is that I did enforce the rules and charged the guest the extra person fee (which I don’t even think they paid; I think AirBnB covered it for them), and they retaliated by leaving me a 3-star review. Because I’m new and have few reviews, this really lowered my rating. Dropped me out of the running for super host and the family/work collections, etc. It will take me a few 5-stars to get my number back up. 
It is really frustrating that you have to weigh the decision on sticking up for yourself and your property vs getting a bad review that could potentially ruin your business.