In case of inclement weather or equipment failure, you may need to cancel all guests on a timeslot at once. This article describes how to do this, and to optionally notify all guests of the cancellation. 

Cancelling Guests

From the Manifest, choose a time in the left navigation pane. From the three dots dropdown, choose "Cancel All Guests..."

A box will pop up to allow you to notify all the customers currently booked on the selected timeslot. You can also add an optional note.