When queuing an online game, PC players can be matched with other players across both PlayStation 4 and Xbox One platforms, while the two consoles can't be matched up together yet. Microsoft, Sony and Psyonix are currently in talks to enable full PS4/PC/Xbox One cross-platform matchmaking.
PlayStation 4 players see PC players with the "PSY NET" icon on the scoreboard and vice versa.
PC players also see Xbox One players with the "PSY NET" icon, but Xbox One players see PC players with the Rocket League shield icon instead.
In addition, the in-game voice chat feature lacks cross platform input/output and only allows communication between players on the same platform. The Quick Chat commands are, however, cross-platform unlike typed and voice chat.
Full three-way compatibility
It is still not currently possible to create a fully mixed-platform party (as of August 19, 2017).
As such, you cannot use the Find Match launcher to queue with friends across platforms against other random players, which requires being able to form a party. Therefore, currently the only way to queue with friends between PS4/PC and PC/Xbox One is to create a Private Match with a name and password that your friends on the other platform (PS4, Xbox One or PC) can enter to join the match.
Private Matches will include AI cars unless the game is filled with invited players.