While the guest is still there, initiate a change request that reflects the actual dates and number of guests for the stay. The guest will be prompted to accept the change, if he does, the Airbnb platform will automatically calculate the price difference. If the guest doesn't accept the change, then address it in the Resolution Center. It is easier for Airbnb representatives to help you if you keep your conversations with the guest in the message thread, so clarify in the message thread your terms and your understanding of the dates involved and number of guests. If it has become uncomfortable or even hostile, then enlist the help of Airbnb sooner by calling an Airbnb support team representative. Do not ask for or accept money outside of the Airbnb platform. If you are only charging the second person for part of the stay, you will need to do it through the Resolution Center. Also, you may want to rethink your policy of charging for extra guests. Instead just price your place to cover two or more.