If Calendar service is configured to allow multiple customer to book for a service slot (e.g. workshop), it is currently impossible to obtain the customer list from Graph API for Bookings, Calendar o...
The response body of 'GET https://graph.microsoft.com/beta/bookingBusinesses/[...].onmicrosoft.com/appointments' is missing the fields called 'BookingAppointmentStatus' and 'BookingCustomerInfoList' from what I could make out looking at the networking of the bookings web app.
The fields are there and just need to be tied to the Graph API.