Can I add an external user as a VIEWER in MS Bookings?

Hi all!

I'm trying to assign an external user (whose email domain is different from mine), but the only role I can assign is GUEST. What I want to achieve is let the external user know the details of each appointment in my Bookings calendar. I believe the role I should assign to the external user is VIEWER. However, I can't assign such role. Any ideas why? is this the expected behavior of MS Bookings? 

Thanks for everything!