Can I play with a friend that is on a different platform?

You certainly can! First, it is important to note what platforms can play together:  

 

 PC

PS4

Xbox One

Nintendo Switch

PC

PS4

X

X

Xbox One

X

Nintendo Switch

X

- Players on these platforms may play together.
X - Cross-Platform Play is not available between these two platforms.

If you and your friends are on compatible platforms, all you need to do to play together is create a Private Match with Name/Password. You may find instructions on how to create and join a Private Match here

Casual and Competitive Play

In order to play Casual or Competitive with your friends, you must create a Party first. Cross-Platform Parties are not currently an available feature in Rocket League.