Tap on the icon in the lower left corner, then tap on Social. You can find your list of friends here, and if you know somebody's Nickname, you can enter it here to invite them. If you switch to the community tab, you can visit and add friends from there.
Additionally, you connect your game to Facebook by tapping on the Connect button in the Friends tab. This will automatically add any friends you might have on Facebook who have also connected their game.
Additionally, you connect your game to Facebook by tapping on the Connect button in the Friends tab. This will automatically add any friends you might have on Facebook who have also connected their game.