Have you tried explaining this to them? That you do not have space for 5 guests?
This morning I received a request for 6 people, although my listing is only for 3 people.
I could only roll my eyes and decline!
Since they already have a booking with you, if they insist you need to contact airbnb and see if they can help you cancel without repercussions for you. Best of luck!
HI Hilary, Have they checked in yet? How many guest do you advertise you are able to host? Nonetheless, If you don't want 5 people simply let the guest know and ask that they cancel the reservation themselves. If they refuse to cancel it themselves call AirBnB to discuss the options.
In our experience, Guests have brought additional unaccounted for people and in one instance we modified the reservation to add on the additional guest fee and in the other the guests did cancel the reservations based on trying to bring 3 additonal guests and 2 dogs! I'm so surprised at what people think they can get away with. At least your booking let you know in advance. Most just try to show up with no advance warning.