This rule automatically sends a notification when an attendee fails to show.
Please go to Advanced Options-->Settings-->Rules and create a new rule (or edit an existing rule) as trigger select "when someone does not show for an appointment") .
In the "What does it do" section please select "send a notification to 1 user(s): click edit :
In this screen please specify what type of communication will you use to alert the user, you can use the tokens (especially [[Appointment.Attendee.Id]] to send the message to this specific user ) as seen above.