Psyonix has released the cross-platform party update for Rocket League. The new system allows friends to create parties regardless of the platform they play on.
The new update brings a new Friends List option to the main menu of the game. In this menu players are be able to manage their friends and invite them to their party regardless of the platform they are playing on.
Cross-platform parties via the new Friends List system was delayed in October. A specific reason for pushing the update to 2019 wasn't given, but it was likely related to getting permission from Sony to support cross-platform multiplayer on its PlayStation 4 console.
The Rocket League Friends Update is now available for all platforms.