If you know more guests are at the property than have booked, then you need to contact the guest straight away.
If your place can sleep six, then you have the option to tell the guest to amend the booking to reflect the numbers who are staying and remind them that they should have made sure their booking reflected the number of guests they have staying.
The best way to make sure that only those on the booking stay are;
1. When you confirm the booking make sure you ask the guest to confirm that only those that are on the booking will be staying
2. Meet guests on arrival
3. Have CCTV - you need to do this in accordance with Airbnb's rules for this (See CCTV/camera's on the Airbnb Help Centre website).
4. Have house rules that state only those who have booked can be at the property.
In this case please leave an honest, factual review.