In December, 2012, the California Supreme Court made it clear that people engaging in an inherently dangerous activity have no recourse against the operator for any injuries or other damages sustained during the activity.
See: Nalwa v. Cedar Fair, L.P.
Specifically, the court held that the only duty of operators, sponsors and fellow participants engaged in a recreational activity with inherent risks was not to increase those risks.
By joining any XEN MTB guided mountain bike ride, you are knowingly assuming the risk. But to set your fears aside, in the 10+ years that Mark Warrick has been leading rides, nobody has died, nobody has sustained an injury greater than a broken bone, and nobody has ever been left behind.