A Refer a Friend program is demonstrated on this site.

The Gather Targeted opt-ins campaign is also shown here too.

There are many ways to add a referral program to your Shopify store. The recommended approach is to create a landing page for members to enrol on the referral program.

Click here to access the demo landing page

A customer can also access the referral program on any page on your Shopify store. This is done by enabling a floating button (as shown below on the bottom left corner of the screen). Also, adding the query param: ?gri=open to the URL loads the referral program as a side bar UI. Click to see this in action.