Hello @Rose170,
You'll have to find out for how many nights the extra guest will stay and agree, in messages on Airbnb.
Then message the guest that you'll send them a request for the extra person charge for each night and point out that they'll have to accept the request for the guest to be allowed to stay.
Then go to your reservations and find the reservation concerned and click on its "Send or request money" link.
Click on "Send money", then on "Next" and then on "Other trip related issues" and "Next". Then fill in the total extra amount and continue the process from there. The guest must accept the money request for it to take effect.
Don't click on "Change dates or guests" as if you enter the extra guest there they'll be charged the extra guest fee for all nights of the reservation.
Steve.