Click the "Invite Friends" button in the main menu to invite your friends to play. You'll see a list of all of your friends' usernames. Select which ones you want to add to the game and hit send. Your friends will receive an invitation via email so that they can accept or decline the request. If they accept, they'll automatically be added to the game.
If you've already invited your friends to play, you can always check to see how many people are currently participating by going to the Settings page.