Distributing and sharing your card to as many users as possible is crucial (unless you want it to be in that case see the article about "Invite only cards").
Distribution methods
QR code
Links
Invite email
User referrals (see details)
Pro Tip: Minimize the number of required fields in your forms. Simplicity encourages more users to complete their registration.
Related
Card Referrals
How to handle referrals to your cards.
Invite Only Cards
How to create cards that are invite only.
Card Registration Forms
How setup the card registration form.
Card Field - Basics
Everything there is to know about card fields.