Appointment created using c# not appears in required attendees

I have created an appointment using sample code in https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/sample-book-appointment 

set myself as organizer and two CRM contacts and two CRM system users as RequiredAttendees , appointment created fine, and i can see it in my outlook calendar but other two users nor contact can see them in there calendars . is there specific way to send appointment requests to required attendees?

I have checked CRM settings - > System settings -> Email tab ,  Appointments Contact and Tasks set to sync to outlook  too.

why other users cannot see the appointment on their diaries ?