You can add friends 2 different ways:
Go to your Game Center app, invite your friends. If they have already launched Happy Street while logged in to Game Center, you should be able to see them on the earth.
Click on the "Add your friends" button in the friends menu, accept to connect to Facebook (Happy Street won't post anything without your authorization). Automatically you will be able to see all your friends playing Happy Street and who are also connected to Facebook.
You can also send requests to those not having the game yet by tapping the add friends button again and sending them invites.
If you are not seeing your friends yet, kill the running app Happy Street
- Double-tap the home button
- Tap and hold one of the icons in the App Tray until the icons start shaking
- In the upper left-hand corner, tap the red circle of the Happy Street app