If a guest wants to bring an extra person to an experience they've booked with you, they can request a change to add someone to the reservation. They can also ask their friend to make a separate reservation.
It's up to you, as the host, to decide if you can accommodate any extra people. If guests show up with extra people, you have the right to turn them away. Airbnb does not offer customer service or insurance to people who don't have an official reservation through the platform, and any liability waivers you may offer your guests won't apply.