EShop supports Notify feature to send the notification email to customers when products come back in the stock.
To show the Notify button for a specific product, you should:
After clicking on Notify me button, a form will be displayed to allow customer to enter their email address to submit to the admin. When product is in stock, the notification email will be sent to that email address.
Go to EShop -> Sales -> Notify to see list of notify requests.
You need to enable EShop Notify Plugin to send the notification email. Please read about EShop Notify Plugin for more details.