If a guest wants to bring an extra person, they should follow these instructions to add someone to the reservation. Or, they can ask the extra person to make a new reservation.
If a guest shows up with extra people unannounced, the host has the right to turn them away. If the experience can be updated to add the extra people, that’s up to the host. However, Airbnb can’t offer customer service for extra people who don’t have an official reservation through the platform, and any liability waivers won’t apply.