Piranha Games has implemented a Co-op Crossplay system that allows players to share IDs and add friends across all digital storefronts and Platforms. However, on specific platforms, you can also use the native friend's list and invite systems to connect for Co-op. You can even use a mix of the 2 invite systems to connect natively with friends of the same platform as well as players on other platforms and enjoy co-op gaming. Presently, however, we are aware of an issue with Steam's native co-op feature. We are working on finding a solution and will patch the game in the coming weeks to address this. In the meantime, you can use MW5's own co-op invite functionality. Finally, for GOG, we won't be supporting its native friends and co-op invite system. GOG players will need to use the Crossplay system we have developed.