If you buy Call of Duty: Infinite Warfare on Steam while your buddy picks it up through the Windows 10 store, heads up. You won’t be able to play the game together online.
This comes directly from a question and answer on Activision’s official support page. Here’s the precise language, for your consideration:
Can I play Call of Duty: Infinite Warfare or Call of Duty: Modern Warfare Remastered for Windows 10 on Windows Store with my friends that are playing on another PC platform?
No, you can only play these titles with other users of Windows 10 on Windows Store.
Make sure you buy Call of Duty: Infinite Warfare on the same platform as your friends.
Not to throw Windows 10’s store under the bus, but I’d recommend going with Steam if you’re buying Call of Duty: Infinite Warfare. Steam will likely be the place on PC that sells the most copies of this game, thus giving it the largest pool of potential online players.
If you can only play with other Windows 10 Store users, you’re limiting your multiplayer possibilities severely. That’s especially if your friends pick it up elsewhere.
Activision did not explain why this is happening.