@Alexia29If a guest tries to check in with more people than have booked, you need to talk to them about it. There's really nothing else to be done once the group is there. You can ask them to pay for the extra guests if you charge for it through the Request money button on the reservation. You can't update the reservation once it's started.
I checked out your listing, and as it's a whole home, it might be worthwhile to get a Ring doorbell or other entranceway camera to make sure this isn't happening. Make sure it's disclosed clearly in your listing. In your rules, let people know that they need to book for the correct number of people who will be staying, and that you don't allow unregistered guests to be on the property. Perhaps also verify the number of guests as part of your welcome message sent a few days before arrival.
If you're having consistent issues with this and are using Instant Book, it might be good to turn it off for a bit and vet guests before they complete their booking. Verify their reason for staying and the number of people in their group before taking the reservation.
