code: def add(self, nickname, email=None, phone_number=None, user_id=None):
          
          member = {
              'nickname': nickname,
              'email': email,
              'phone_number': phone_number,
              'user_id': user_id,
          }
          return self.add_multiple(member)
docs: Add a user to the group.
  
          You must provide either the email, phone number, or user_id that
          uniquely identifies a user.
  
          :param str nickname: new name for the user in the group
          :param str email: email address of the user
          :param str phone_number: phone number of the user
          :param str user_id: user_id of the user
          :return: a membership request
          :rtype: :class:`MembershipRequest`