What happens if the guide decides someone isn't able to safely join the tour?

Your guide's first job is keeping everyone safe, including you. If, at the start of the tour or during the initial briefing, the guide judges that someone doesn't have the skills needed to safely handle the bike on that specific route, they may ask that person to sit the tour out. Because the decision is about safety rather than a change of mind, no refund applies in this situation — so it's worth being honest with yourself about your comfort level before booking.