Write to many guests, once each: everyone confirmed in a class session (target.session_id), starting on a local date across offerings (target.date), or booked ahead on an offering, or picking another time of it (target.offering_id). Returns how many were sent, skipped (erased guests) and failed; one that would pass the daily email limit is refused whole. Reference: https://daybooked.io/docs/api#messages_broadcast