You can send Duplicates of your Silver cards to your teammates.

Here's how to send cards:

•  When you see a card request in the team chat, tap the Send button
• In the window that opens confirm the action

Please note:

• It's not possible to send duplicates of Gold cards.
• If neither of your sets has duplicate Silver cards that you could share, you won't see the request in the team chat.
• You can send Duplicates from both completed and incomplete sets. You can still share Duplicates even after you've completed the whole Collection.
• If you aren't part of a team yet, check this article on how to join a team.