There are two places in Flybook where you can export client email lists for import into an email marketing platform.
Option 1
Navigation: Reports > Miscellaneous > Client Export
From here you can "Export All Clients To CSV" or select filters (eg. specific Activities, Rentals, etc) and a date range then click "Apply" to see how many clients those parameters will include then click "Export These Clients To CSV?"
Option 2
Navigation: Reports > Miscellaneous > Client Participation
From here you can use the filter to narrow what clients participated and utilize the date range to target a specific group of customers. This will export to CSV when you click "Apply."