Hello! How many people did the guest list when they made the reservation? I think, even with instant bookings, they cannot list more than the room will allow as maximum. Go and see how many guests they listed. This is their legal contract with you. If it says more than 2, immediately change this in your listing on airbnb. If they only listed 2 people, in their reservation, explain that is all you are prepared for. By the way, you dont Have to make up the futon, if you dont want to. They reserved one room for 2 people. If another joins them,..so be it. They can sleep in that One room, on the floor. They only rented one space from you. Dont charge , and give them a pillow and a blanket, and show them the floor. They are trying to pull a scam on you. You could then say, if they want the futon, there is a $30 per day fee. Let them decide. You may not have listed that you charge for an extra guest, but you also didnt list anything more than the one room they booked. If they all want to crash in there together to save money,...let em lol.