First and foremost, anybody playing Call of Duty: Modern Warfare needs to have a Call of Duty account. You can go here to create one using your PSN, Xbox Live, Steam, Activision, or Battle.net account. Once you get you get your hands on the game, you choose to opt in or opt out of crossplay. Choosing to enable crossplay means players can play with others on PlayStation 4, Xbox One, and PC. Once you have your Call of Duty account and you enable crossplay, you can create cross-platform friends lists and parties comprised of players from PS4, Xbox One, and PC.
One of the largest questions fans have had about this feature is how Infinity Ward will balance keyboard-and-mouse players with controller players. PlayStation 4 players can decide to use a keyboard and mouse. PS4, Xbox One, and PC players can all play with each other as long as they all use controllers, or all use keyboard and mouse. However, you can also join lobbies where there are no control-scheme filters, meaning you can play with players across all platforms with controllers and keyboard and mouse.
Infinity Ward has also announced that it intends to demonstrate its commitment to this crossplay initiative by delivering most post-launch content (maps, modes, Spec Ops missions, etc.) simultaneously across all three platforms. Despite this, the franchise maintains its partnership with PlayStation, with the studio stating that PS4 players will have “an exciting Day 1 advantage.” The team did not provide details about what that means, but says information will arrive soon.