A first reason why the total number of Joyn customers is higher than the total number of receivers of your emails, is that not all Joyn users register their card.
To send emails, you need to know your customers' email address. That's why you should always encourage your new Joyn users to register, or even better, to use the Joyn app on their smartphone.
If possible, when handing out a Joyn card to a new user, try to enter your customer's email address in the iPad yourself. The only thing left to do for the customer, will be to confirm that email address via the email they’ll receive from Joyn.
A second reason why the number of receivers is lower, is that customers can unsubscribe for email campaigns. So the challenge is to make your emails as interesting as possible, so no one even thinks of unsubscribing.