I'd send a message now going something like: "Hey, I'm checking in to say I'm glad it seems your trip is enjoyable so far! Also as a heads up, my listing states extra guests above 6 cost $100 a night. I'm happy to adjust your booking to reflect this, so please keep me posted if the other members of your party will be sticking around overnight. Hope you're having a great trip--as always, let me know if I can do anything to make it better!" They're less likely to trash the home of a host who's been nice to them, and if they're going to trash the place at all, then there's not much that can be done at this particular moment.
Also... I'm going to hope you've disclosed that you've got a CCTV within your listing? Air is probably going to be more strict about that, than siding with you on extra guests since they make it clear hosts have to notify guests about any and all cameras. So if nobody knows of its existence, then you going to Air and saying you've got footage is sort of like a police officer bringing in evidence obtained without a warrant.
Anyway, sorry that you're in a delicate and tricky position!