Hi @M-11
I think your chances of getting extra payment out of these guests is very slim.
Airbnb require photographic or video evidence - which is why many hosts have survelliance at the entrance of their properties ( which MUST be disclosed in the house rules)
However, even better than catching guests doing the wrong thing, is preventing them doing the wrong thing to start with. I would tighten up your house rules to say something to the effect that the names of everyone staying must be stated on the listing. NO extra guests or visitors permissible without prior arrangement. Guests violating this rule will have their stay immediately cancelled.
Something along those lines.
Even if you DON'T have a security camera, it's worthwhile stating that you do in the relevant section. Guests who plan in advance to bring extra guests seek out listings with self check in and no security cameras.
And finally, please do be sure to leave an appropriate review for these guests. Keep it short, unemotive and factual. I"m not great at writing reviews, but you might say somethign like.
XX were pleasant to communicate with ( if they were) and left my listing tidy( if they did).
xx booked and paid for 2 guests, but brought an extra 7 guests without my permission or any extra payment. I could not recommend xx to future hosts.
Other people on this forum can probably word it better and offer further advice.
Regards