In Friendly Mode, you can play with any of your in-game friends in CLASSIC or RUSH mode formats.
To play friendly games, tap on the Friends tab in the bottom right corner of the main lobby screen.
Tap on Battle and choose game mode (Classic or Rush) to play.
Then you will prompted to select which friends to play with and the tour(course) to play on.
If the friend(s) you invited accept your invite, then the game will begin.
Keep in mind, you don't need all 8 players for Rush mode in friendly games!
You will not lose trophies in friendly games either!