Facebook
Sign into Facebook and Open up SimCity BuildIt, then tap on settings icon And then Tap on the Connect to Facebook button.

Gamecenter
This allows you to track achievements an also saves your progress for use on other iOS devices.

Google Play
Also on google play you can track achievements An save all you progress for use on other Android devices. For this connect to your Google Account (Gmail) to both services. In order to add friends you Open up Google+ And Add one of your friends emails and send them requests. Create a "Circle" on your Google+ account and name SimCity as example. Now add these friends to your 'Circles' as one of your neighbors. Open your game and your friend will now appear as a neighbor after you have connected google play and signed in through your game.