ReferenceField('members',
   ... 
),

class Group(BaseContent):
    ...

    security.declareProtected(View, 'getMembers')
    def getMembers(self):
        members = self.getField('members').get(self)
        members.sort(lambda x, y: cmp(x.employee_last_name, 
                                      y.employee_last_name))
        print members
        return members