By default, card referrals are enabled (unless the card is configured as "Invite only"). Card referrals makes it easy for any of your users to allow other users to get your card.
Screenshots
Apple Wallet
On Apple Wallet the referral link is on the back of the wallet pass. The back is accessed by opening the wallet pass and then, in the top-right corner, click on the "three dots", choose "Pass details" and finally click on "Refer a friend".
Google Wallet
On Apple Wallet the referral link is on the back of the wallet pass. The back is accessed by opening the wallet pass and then, in the top-right corner, click on the "three dots" and finally on "Refer a friend".
Related
Invite Only Cards
How to create cards that are invite only.
Card Registration Forms
How setup the card registration form.
Card Anatomy
Learn the basic anatomy of a card.