You can automatically notify the booker of any un-assigned attendees left on their booking by using the Un-assigned Attendees event trigger.

This is found within the Auto Response panel on a Message Campaign:

Control > Promote > Message Campaigns > (Add/Edit) > Auto Response


Here, you can set up auto messaging and use the message delay option to decide when to send the reminder after a customer has purchased an event ticket.

Please consult the Responsive Message Campaign Guide (under Create Message Campaign) for more information.