When you select Online multiplayer, you can chose to quickly jump into a random game, or you can find and play with people in your Friends list.
To play with your Friends, one of you needs to first join an online match. The easiest way to do this is to go to Online / Create Match. You can choose:
Public: Anyone including strangers can drop into your game.
Private: Only people in your Friends list can drop into your game.
Once you have set up a match and are in the lobby, your Friends can join.
To join your Friend who is already in an online match, go to Online / Find Friends. After you pick your vehicle you can see a list of any Friends who are currently playing, and join their game.
Please note! Riptide GP: Renegade uses different game servers for different territories, so if your Friends are in another country it's possible they may not be able to play on the same server. This is something we are considering modifying in the future.
For instructions on how to add friends in Google Play, see https://support.google.com/googleplay/answer/3263264?hl=en/