Modifying EKParticipants (attendees) in EventKit like Sunrise

Swift version of rebello95's solution. I can confirm it works like a charm! I played around with the firstName and lastName properties but they do not seem to matter at all, so I left them out of my example. Only the emailAddress matters, which is matched against your contact list to display a full name in for example the native calendar app.

private func createEventWithAttendees() {
    let eventStore = EKEventStore()

    let calendars = eventStore.calendarsForEntityType(.Event)

    let event = EKEvent(eventStore: eventStore)
    event.title = "TEST EVENT"
    event.startDate = NSDate()
    event.endDate = NSDate().dateByAddingTimeInterval(60 * 60)
    event.calendar = calendars[0]

    var attendees = [EKParticipant]()
    for i in 0 ..< 5 {
        if let attendee = createParticipant(email: "test\(i)@email.com") {
            attendees.append(attendee)
        }
    }
    event.setValue(attendees, forKey: "attendees")

    try! eventStore.saveEvent(event, span: .ThisEvent)
}

private func createParticipant(email email: String) -> EKParticipant? {
    let clazz: AnyClass? = NSClassFromString("EKAttendee")
    if let type = clazz as? NSObject.Type {
        let attendee = type.init()
        attendee.setValue(email, forKey: "emailAddress")
        return attendee as? EKParticipant
    }
    return nil
}

I figured it out!

Essentially, one must go into the EKAttendee class, which inherits from EKParticipant. I did this by creating a generic instance of that class using the NSClassFromString() method.

Once you have an attendee, you can set the properties using the setValue:ForKey: function.

Lastly, compile your EKAttendee instances into an array, and set that to the EKEvent's attendees property.

I tested this with an Exchange account on my device, and it worked like a charm - invite sent and everything!

The code below is what I'm now using to set the attendees of events. My example is for creating new events, but this can be done very simply for existing events by making a copy of the attendees list on the EKEvent, then modifying that and re-setting it.

//Create generic event info
EKEvent *event = [EKEvent eventWithEventStore:database];
event.title = @"TEST EVENT";
event.location = @"Test location";
event.notes = @"Example notes";
event.startDate = [NSDate date];
event.endDate = [[NSDate date] dateByAddingTimeInterval:(60 * 60)];
event.calendar = exchange;

//Do our super clever hack
NSMutableArray *attendees = [NSMutableArray new];
for (int i = 0; i < 5; i++) {

    //Initialize a EKAttendee object, which is not accessible and inherits from EKParticipant
    Class className = NSClassFromString(@"EKAttendee");
    id attendee = [className new];

    //Set the properties of this attendee using setValue:forKey:
    [attendee setValue:@"Invitee" forKey:@"firstName"];
    [attendee setValue:[NSString stringWithFormat:@"#%i", i + 1] forKey:@"lastName"];
    [attendee setValue:@"[email protected]" forKey:@"emailAddress"];

    //Add this attendee to a list so we can assign it to the event
    [attendees addObject:attendee];
}

//Finally, add the invitees to the event
[event setValue:attendees forKey:@"attendees"];

//Save the event!
NSError *error = nil;
[database saveEvent:event span:EKSpanThisEvent error:&error];
if (error) {
    NSLog(@"Save failed with error: %@", error);
} else {
    NSLog(@"Success!");
}