Add attendees to the parent event

        :param attendees: list of attendees to add
        :type attendees: str or tuple(str, str) or Attendee or list[str] or
         list[tuple(str,str)] or list[Attendee]

def add(self, attendees):
        """ Add attendees to the parent event

        :param attendees: list of attendees to add
        :type attendees: str or tuple(str, str) or Attendee or list[str] or
         list[tuple(str,str)] or list[Attendee]
        """
        if attendees:
            if isinstance(attendees, str):
                self.__attendees.append(
                    Attendee(address=attendees, event=self._event))
                self._track_changes()
            elif isinstance(attendees, Attendee):
                self.__attendees.append(attendees)
                self._track_changes()
            elif isinstance(attendees, tuple):
                name, address = attendees
                if address:
                    self.__attendees.append(
                        Attendee(address=address, name=name, event=self._event))
                    self._track_changes()
            elif isinstance(attendees, list):
                for attendee in attendees:
                    self.add(attendee)
            elif isinstance(attendees,
                            dict) and self._cloud_data_key in attendees:
                attendees = attendees.get(self._cloud_data_key)
                for attendee in attendees:
                    email = attendee.get(self._cc('emailAddress'), {})
                    address = email.get(self._cc('address'), None)
                    if address:
                        name = email.get(self._cc('name'), None)
                        # default value
                        attendee_type = attendee.get(self._cc('type'),
                                                     'required')
                        self.__attendees.append(
                            Attendee(address=address, name=name,
                                     attendee_type=attendee_type,
                                     event=self._event,
                                     response_status=
                                     ResponseStatus(parent=self,
                                                    response_status=
                                                    attendee.get(
                                                        self._cc('status'),
                                                        {}))))
            else:
                raise ValueError('Attendees must be an address string, an '
                                 'Attendee instance, a (name, address) '
                                 'tuple or a list')