Good question, Patrick.. this comes up a lot. The best approach is to be transparent and flexible upfront. Let the guest know that your listing rate covers two guests, and that if a third person joins later, you'll simply update the reservation to include the extra guest fee.
You can also remind them that Airbnb requires accurate guest counts for insurance and security purposes, so it's important to update the booking if plans change. This way you protect yourself while keeping the tone friendly and professional.