The leader may check to see if everyone is suitably attired and seems capable of doing the walk in question.  If the leader thinks that someone is ill-prepared or does not seem capable of completing the walk, he or she has absolute discretion in refusing to allow that person to take part.   This is more likely to arise in Hard or Strenuous walks where such a person might put others at risk. The leader should appoint someone to act as a back marker.