Hello @Amanda-and-Padraig0,
One way would be to use the "Serd or request money" feature in the resolution centre. You could ask them for your extra guest fee for 2 nights.
You could message the guest in advance saying that you're going to send a request for the extra guest for two nights so they know to expect the request.
Click on "Host" and then on "Your resservations". Then find their reservation and click on the "Send or Request Money" link in it.
Then click on "Request money" and then on "Next". Then follow the sequence from there.
If the guest accepts the request you'll be paid. If they don't, then you'll have to decide about the extra guest's access.
Steve.