As it stands, Call of Duty: Infinite Warfare and Modern Warfare Remastered are available on the PC and can be purchase via a number of ways, such as Steam. However according to Activision’s FAQ page, it clearly states that Windows 10 Windows Store gamers can only play with each other. It says, “No, you can only play these titles with other users of Windows 10 on Windows Store.”
This means that if you were to purchase the game from Steam but your friend buys it from the Windows Store, you guys won’t be able to play together. According to the folks at Windows Central, they heard from a source that this isn’t so much Microsoft’s restriction, but Activision who decided to segregate Windows 10 Store and Win32 PC platform players.
Microsoft has since released an official comment to Windows Central which seems to confirm that, and it reads, “We support cross-play between devices and platforms for partners who want to enable it.” We’re not sure why Activision made this choice but it is certainly a weird one and hopefully one that they will reverse soon.