You don't let them in @Natali24
Speak to the person who booked. Either ask Airbnb to cancel the booking as (presumably) they broke your house rules by bringing more people than they had booked.
If you have the room you could prepare the extra rooms only after you had ensured the guest amended the booking for the extra guests.
Personally if someone brought that many extra guests I would ask Airbnb to cancel the booking as they brought in six extra guests.
How did you handle it @Natali24
EDIT
Just checked your listing and it says absolutely nothing in your house rules about extra guests not being allowed on the premises Add this now.