In case you have connected the game to your Facebook account, you're able to send invites to your Facebook friends from the pop-up that appears once a day when you launch the game and enter the map screen. 

All Facebook friends that are already playing the game and have the game connected to Facebook will automatically appear in the game when you're also connected, so you do not have to send invites to them to be able to see their scores or to swap lives with them.