CoD: WaW How to play with 3-4 players in zombie mode?

So my friends and I have been trying to play with 3 players in zombie mode and cant figure it out. It says its up to 4 players online but we cant figure how to do it, does anyone know how?CoD: WaW How to play with 3-4 players in zombie mode?
Invite them so your all a party then enter it all at the same timeor click invite party.CoD: WaW How to play with 3-4 players in zombie mode?
go to cooperative and go to splitscreen have everyone sign in then choose the level and there is a campaign bar and below it a nazi zombie bar, choose that, you also need to beat campaign to unlock it.
No you can't, its only 2 offline, its not 4 people unless you do it off line as well
you can only have two players

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