How do I play Riptide GP2 with my friends?

Riptide GP2 supports 3 different multiplayer options, although not all of these are supported on all platforms.  
 
  • Online Multiplayer - Log in with your account and play with others over the Internet.  You can play with randomly matched opponents, or with Friends using the Invite feature.  
    • Supported Platforms:
      • Android (using Google Play Game Services)
      • iOS (using Game Center)
      • Steam (using Steam ID)
  • VR Challenge - Race against ghosts of your friends as you try to set the best time on each track.
    • Supported Platforms:
      • Android (using Google Play Game Services)
      • iOS (using Game Center)
      • Steam (using Steam ID)
      • Amazon (using Game Circle)
      • Windows Store (using Google Play Game Services)
      • Xbox One (using Xbox ID)
      • PlayStation4 (using PSN)
  • Split Screen Multiplayer - Race against multiple friends on a single device!  Connect 2 or more Bluetooth or USB game controllers (or a Smart TV remote control) to your device, and run the game. The screen gets divided into multiple small screens, one for each controller.
    • Supported Platforms:
      • Android
      • Steam
      • Amazon
      • Windows Store
      • Xbox One (supports up to 6 players!)
      • PlayStation4