Yes, Killer Klowns from Outer Space offers crossplay across all platforms, both with your friends and through online multiplayer.
To invite your friends from any platform, just head to the Friends menu and then the Party Management tab. From there, you'll need to select your Privacy Settings and toggle it on over to 'Public,' where you'll find a party code that you can share with your friends so they can join you.
Once you're in a party with your friends, everything should go smoothly from there - features are pretty much the same across all platforms, so everything is compatible across all of them. You'll notice a little icon next to anyone's name who's on another platform, just to let you know that you won't be able to directly communicate with them in-game the same way you would with players on your own platform.
In public matchmaking, crossplay is on by default, and turning it off can result in longer queue times, so we don't recommend it - however, it's up to you whether you want to or not. If you want to toggle crossplay on or off for any reason, you can do so in the Gameplay section of the game's Settings.