We don’t have this feature in place.
Reason is some studios have 2000 to 4000 customers and emailing them can really take a lot of resources.
You can do 2 things:
1) Create a 1 class in the far future. Add all your customers to this class. Send email to this class which will email all customers.
2) Export all your customers into CSV. Create a Mailchimp free account and import the list as subscribers. You can manage your mailing list on Mailchimp.
See more on how to Export your customers https://software.kriya.com.au/docs/how-to-filter-and-export-customers/