If you need to add an extra guest fee, the ability to do that is under the Pricing Settings tab found in the calendar view. At the bottom of the pop up is an arrow to open the Extra Charges and Currency section. Add an extra guest fee there. This will apply to all future reservations.
If you need to charge a guest right now for an extra person now, you can do this from the Resolution Center.
If your problem is something else, you can ask for advice here and we might be able to answer it for you. Otherwise, use the Search box and look for "Contacting Airbnb: A Community Help Guide"