@Penny55
Penny - agree the additional cost for the entire period with the guest making the booking. Then, ask them to book and tell them you will then send them a revised price for the new amount. Remind them that as AirBnB charge a 3% booking fee, it will be the total of the new accommodation charge, plus 3%.
You then go to Your Reservations, find theirs and click on Change/Cancel. Go to Change and you will see the existing price. Overwrite this with the new price which should be the old price, plus your additional fee for the extra guest, plus any cleaning fee. Do not add the AirBnB service charge as this will be done automatically. Your guest will then need to accept the revised offer - or else you will just be paid the original amount - so make sure they are aware that they need to 'accept' the new price.