Note: Classic Outlook is going away in 2029 so this article will eventually be removed.
When adding users to a shared mailbox, it may take a few minutes before it populates on the user's end. Wait a bit and then close and restart Classic Outlook again.
If that doesn't work, then manually add the shared mailbox to Classic Outlook.
Launch Outlook.
Select the File tab on the ribbon, then select Account Settings > Account Settings.
Under the Email tab, ensure the correct account is highlighted, then choose Change.
Choose More Settings > Advanced > Add.
Type the shared email address.
Choose OK > OK.
Choose Next > Finish > Close.