Hi All,
I recently had success with this, so hoping this tip helps. I have a Mac and have never seen the option to only send the invite to added attendees in the outlook app. I logged into outlook online, and opened the invite, went to scheduling assistant, added an attendee, and clicked send. Once I clicked send, it gave me the option to only send to added attendees. Not an ideal user experience but it got the job done! Note - I think if you make any other changes to the invite outside of adding attendees, it will remove the option to only send the invite to added attendees.