Hello!
I`m quite new to this, but I managed the same problem (my wife and I have a B&B villa with 6 renting rooms) by charging every guest above 2 with half of the basic price.
Since each of our room can host 2 people the basic price is set per room per night. If there is a party of lets say 8, they pay for three more rooms per night.
So if I`m correct (we have an account for only two weeks now and the season starts in spring - so we have some time to prepare ourselves) the pricing for 8 for 5 days would be:
34€ per room * 4 rooms (8 people) * 5 days = 680€ + Airbnb fee
But that aplies only for parties larger than 2. If there is a couple, they get the normal nighlty pricing per room.
What do you think?