I have to disagree with Dave's answer - it's spot on consistent with Airbnb's response to this issue, but I don't feel it's consistent with what Airbnb claims to be offering.
The problem of guests booking for the wrong number of people and then not responding to requests for altering the booking seems to be growing, and it's our understanding that part of what Airbnb is providing in exchange for their fees should include only requiring hosts to honor accurate bookings - functionally, we wonder what the difference would be between this situation and a situation where a guest said, "Well, I booked the room for Thursday, but I'll actually be staying Friday". It seems hard to imagine Airbnb would ask a host to choose between honoring that booking for Friday at Thursday's price and losing Friday's income, or taking a hit to their metrics.
If hosts have to use one of the three "get out of jail free" cards that you get per year with Instant Book ("I'm uncomfortable with this guest") every time a guest refuses to alter an inaccurate booking, it puts the burden on hosts to decide whether to open themselves up to potential liability for someone who isn't on the booking, or take a hit to their metrics.
For hosts who are concerned that Airbnb is demonstrating more of the kinds of corporate practices that some folks worry about - lack of transparency, binding arbitration, heinously dense terms of service - it's really troubling to see Airbnb taking the attitude that it's the host's problem to choose between liability and being penalized in search results.
Full disclosure, we just had this issue happen, and even though Airbnb agreed to cancel the booking, whatever cancellation method they used still allowed the guest to rebook a few hours later, so if you have this issue (where the guest doesn't respond to Alteration requests), unfortunately your best bet is to call Host Support and have them reach out to the guest, and then you can save your "get out of jail free" card for that guy who promises to follow all the rules but just looks like an axe murderer. 🙂 Miraculously, even though this guest somehow didn't receive any of our messages or texts, she was able to see almost immediately that the reservation was canceled. To say that we're angry Airbnb structured their system to allow her to rebook with us would be an understatement.