If the waiver must be signed by the parent or legal guardian, then the leader of the youth group cannot sign the waiver.  The waiver is ineffective unless signed by a parent or legal guardian unless the parent or legal guardian has authorized the leader of the youth group to act as the agent of the particular parent or legal guardian.  The authorization to act as the agent of the parent or legal guardian should be in writing and should be very specific as to allowing the agent to sign your particular waiver.  The authorization should be signed by the parent or legal guardian.  Without such a written authorization as the agent of the particular parent or guardian, your rock climbing gym will be liable.