At the moment you can not add friends but will be available in the nearest update. After update you can add friends with your Facebook, to sell goods on the market/bazaar.
Register on Facebook.com and log in with your Facebook account to attach the game you want (in the Menu)
Add a person you like to Friends on Facebook
Your friends that plays Silk Road will automatically showed in the Friends List. Now you can make deals with them on the game market.
We will add more ways to interact with your friends in the future.