There is a bit of a strange situation for Call of Duty: Infinite Warfare on PC. There are currently two versions of the game, one for Steam and one for Windows Store, and puzzlingly, the two don't provide cross-play between one another.
Some thought this might have been Microsoft trying to give Steam the cold shoulder, but it looks like this is an Activision decision. According to Windows Central, who reached out to Microsoft, they got word that more or less entirely put the blame on Activision. They said:
We support cross-play between devices and platforms for partners who want to enable it.
Hopefully Activision see sense here, and don't leave players who bought the Windows Store version flailing in the wind.