If a guest wants to bring an extra person to an experience they've booked with you, they need to either cancel their reservation and rebook with the correct guest count or 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.