You can set up a promotion for people with certain email address, for example for addresses ending in @gmail.com for.
In Customer Order Setup -> Partners Promotion Maintenance -> Create new
This will take you to the below screen.
The Promotion Page name will form part of the URL
The Promotion Page Description will be used on the screen where customers can validate their email address
Email Validation Rules will be the section of the email address every customer receiving the discount code must have, for example in this case (not reccommended) every user with an address ending in @gmail.com will receive the discount.
Once this screen is completed, nagivate to https://[URL of club]/discount/[Promotion Page Name]/
In this case the URL will be https://[URL of club]/discount/This-is-a-test/
This is the URL the customer group receiving the discount will need to go to.
Following the link above takes the customer to the below page
In which they can validate their email and password. A discount code will be emailed to the customer. The customer must order using the same email as they entered here