On May 24th at 6PM EDT, Rocket League players on Xbox One and PC will be able to play together via cross-play. This will add about 2 million potential players to the pool of players on PC, although still to be determined is whether cross-platform parties will be supported.
“With this new addition, players can now easily include (or not include) cross-network rivals in both public and private matches, thanks to an easy-to-find toggle check box in the Options menu,” notes the announcement.
This is the first Xbox One game to support cross-platform play with Steam. Other games, like Killer Instinct, have supported cross-play between Xbox One and Windows 10 directly.