I have a request from a potential guest who says a friend is coming for one night of the four days she is booking and would like to know (me too) how this is to be charged. The Airbnb software does not seem to have this flexibility. I charge for one guest and $20 per night for the second guest. I have twin beds. Of course, this is easy if there are two guests for all four nights. It would also be easy to take cash for one night but that may not work for Airbnb.
It can be done in the Res Center or through a special offer.
Yes, as felixcat says, just calculate what the price would be - 4 nights for 1 guest, add the $20 for the 1 night of the extra guest, go to ‘change or cancel’ and enter the new price. They will have to accept it on their end - and then you’ll get another ‘congratulations you have a booking’ message.