Hello @Corinne211
In this situation you can either ask Airbnb to cancel a booking if you know your guests have brought in additional guests to stay contrary to your house rules.
Or you contact the guests and say you can see that they have XXX guests staying in your property who they haven't booked and paid for (presume you live on the premises or have CCTV) and that you are sending them an amendment to cover the cost of the extra guests. I would say to guests that if the amendment isn't accepted unfortunately the booking will need to be cancelled.
If you didn't have an extra charge on your listing for extra guests you can't add it and then charge your guests as it wasn't in force at the time they booked.