Of course! Just click on the "Invite friends" that appears on the far-left side of your screen.
You will see a window where you get a link by clicking "Invite". Just send the link in whatever way you want to your friend (e.g. if your friend is online on Facebook chat or Skype you can send it there). When your friend clicks the link, he will enter the game and you can start playing together. It's that simple!