@Gavin---Vida0 , sorry this is happening to you. It shouldn't! your house rules state that there is a $24 charge per person after the first two. so as soon as the guests arrive and you notice that, you need to be assertive and not let them in before they agreed to include the extra people into the reservation by going through "change reservation"'and change it from 2 to 6 people. Be firm, know how to get to that page, do the change right then and there,in front if the guest have them agree to it right then. problem solved and Airbnb will automatically compute the extra payment due to you. sorry you missed that 3 times.
It sure is hard and uncomfortable to be assertive right when it happens, but it's your house and you are in charge of it. You can also get pro active by communicating with the guest before arrival and make sure they and you are on the same page.simply ask or bring up the subject in a message, like "the booking says you are coming with 2 people, is that correct? " It would take some nerve to blatantly lie at tgat point. Also, put it into your house rules that only the people listed on the reservation are allowed as guests. simple and effective. then you have then they are breaking the house rules and you have the right to refuse them. Good luck next time!