People on my team are exchanging cards to complete sets but I can't find any cards to exchange even..

On the bottom right of screen, go to collection and click . On bottom half of screen you will see icons for 15 collections. Click one of these. Each of these has 9 cards. Cards you have are flipped up and colored. Cards you don't have show only back of card. Cards you have extra of will have a +1, +2, etc on their top left if you have extras. To send extras click on card, hit send , and then you can choose who to send. You can only send to teammates or friends.