Head to the Credits section of your account. Below the credit balance circles, you'll find a "Want to send your credits to another account?" link. Click it, search for the recipient account, and send.
If you sent credits but your friend didn't get them
Most common cause: the credits never left your account. Two scenarios to check:
Either way, head back to your Credits page, double-check your balance is intact (it usually is), and re-send to the right address.
If you're certain you sent to a specific account and they never received them, open a support ticket with the recipient's email/username and the date, and we'll trace it.