private void copyAttendees(CalendarEventModel eventModel, Map<String, Object> graphCalendar) {
    List<CalendarAttendeeModel> attendees = eventModel.getAttendees();
    if (attendees == null) {
      return;
    }
    List<Map<String, Object>> graphAttendees = new ArrayList<>();
    attendees.forEach(
        attendee -> {
          Map<String, Object> graphAttendee = new HashMap<>();
          graphAttendee.put("type", attendee.getOptional() ? "optional" : "required");

          HashMap<String, String> emailAddress = new HashMap<>();
          emailAddress.put("address", attendee.getEmail());
          emailAddress.put("name", attendee.getDisplayName());
          graphAttendee.put("emailAddress", emailAddress);

          graphAttendees.add(graphAttendee);
        });

    graphCalendar.put("attendees", graphAttendees);
  }
Maps from a transfer event type to a Graph API event resource as defined by:
https://developer.microsoft.com/en-us/graph/docs/api-reference/v1.0/resources/event.
extensions/data-transfer/portability-data-transfer-microsoft/src/main/java/org/datatransferproject/transfer/microsoft/transformer/calendar/ToGraphEventTransformer.java