In the list of cards, scroll to the card you want to share, and click the 'share' icon on the bottom right corner of that card.
You can also share a card directly from your homescreen using the Haystack widget
And of course, you can use the 'share' button while looking at the card in full screen view.
Haystack lets you share the card via text, email, QR Code, NFC, Facebook, Twitter, and much more.