After struggling to get this to work all day I can finally validate a work around to anyone experiencing game share problems!
Requirements: 2 controllers
Step one!
Make sure both the account that purchased the game, and the account you’re trying to share with are on the same console
Step two!
Start diablo 4 on account that purchased the pre order
Step 3!
If a character has already been made, must create a new character on account that has purchased the game, at this point before starting turn on second controller and sign into account you want to GameShare with (should get a message stating player 2 can join once player one starts the game)
Step 4!
Once new game has started on account that purchased the game, join in as player 2 on the 2nd controller
Step 5!
Leave game on controller player one was signed into, and bam! It lets you stay!
Edit: Twitch
Link to my twitch if you need proof or live help!
Also shoutout to perendia for discovering if you don’t have a second controller you can use your phone with the ps remote app!