Blizzard have jumped in to clarify how Game Sharing works for couch co-op in the Early Access period of the Diablo 4 launch.
Game Sharing
Users who purchase a Deluxe/Ultimate Edition on an account should use that same account to host couch co-op sessions during the Early Access period. This ensures the guest player can join and participate in co-op.
Accounts that weren’t the original purchaser of Deluxe/Ultimate Edition of the game on that console will not be able to play in Early Access unless they join as a second player with the account that made the original purchase.
You can read the entire official news post HERE
