Use Case for Adding an Attendee to an Event
Use the PATCH command to add an attendee to an event.
          PATCH {{REST_SERVICES}}/record/v1/calendarevent/<id>/
{
    "attendee": {
        "items": [
            {
                "attendee": {
                    "id": "5"
                },
                "response": "ACCEPTED"
            }
        ]
    }
}