Hello, I’ve had this problem several times. If you go on their reservation click details scroll to the almost bottom where it says request money. You can request the amount of money they owe that way it all goes through Airbnb. Air Bnb will send a message to your guest saying they owe you money. Ask for the exact amount of money that they owe you for the extra guests. If they do not pay, I believe they Will not be allowed to make another reservation until they do.
I would request it while they are there and then let them know that you requested it on the site for the extra or additional people.