You have two choices - you can go into the reservation (even with them staying there) and submit a change. Update to the correct number of people and then enter the new price. Be careful. Look at your payout and write it done before you do this. It will ask for a new payment amount and you want to make sure the new payout is correct (sometimes it adds the cleaning fee, sometimes it does not). So, for example, if the current payout is $325 before Airbnb deducts its fees, then add the extra people for the total nights and add it to the $325 ($325+$30+$30 for example for two nights). It will be a reservation change request which the guest may agree with, or they may not. If they ignore, get Airbnb involved and send a bill to them through the resolution center for the extra people. But first see if you can get them to take the reservation change with the new payment total first.