Hey there @FieldSched -- great question!
You are correct. Firstly -- with Calendly, anyone that books with you is considered an “invitee.” Whether or not they actually show up for the booking will not change this. I just wanted to clarify for you!
When setting up and applying a Workflow to an event type, that workflow will be sent to every invitee that books the event regardless of whom they are.
This means that, if you set up a workflow that is meant to fire off immediately upon booking, all invitees that book will receive it. If you set up a workflow that is meant to fire off immediately after the booking ends, that will go to all invitees booked after the event takes place.
At this time, there is no way to customize this workflow process. I apologize!
I hope this helps!