You can easily add friends by going to the Friends & Family menu and choosing either of the three available options:
  1. Contacts - Will ask for your number and will automatically add friends that are playing Monopoly GO from your contact list
  2. Facebook - Will ask for your permission to connect the game to Facebook and will automatically add your Facebook friends that are already playing Monopoly GO
  3. Invite - This will allow you to send an installation link to any of your friends and have them install the game. They will automatically be in your Friends list the moment they play & Install the game

Please keep in mind that the inviting new friends feature is only available for installs coming from IOS and Android.