Maybe a user-creatable discount/coupon code and e-mail field for the friend? E.g., something like: "Hi, your friend [Jane Smith] is a user of [Service XYZ] and

Maybe a user-creatable discount/coupon code and e-mail field for the friend? E.g., something like: "Hi, your friend [Jane Smith] is a user of [Service XYZ] and is inviting you to use it! Use the following coupon code to get started! www.yourservice.com/random-hash/