The Friend Pass is now available on Steam, so you can play with a friend even if they don't own the game!
Regardless of platform, as long as one of you owns the game you can play!
Make sure to download the demo of the game, have the player that owns the game generate a game code, and then save the world together!
More information here: http://www.operation-tango.com/operation-tango/faq/
Need a player 2? Check out our discord! https://discord.gg/cvq7Zua