After your admin has added you as a member of a shared mailbox, close and then restart new Outlook. The shared mailbox should automatically display in your Folder pane.
If your admin only recently added you to a shared mailbox, it may take a few minutes before the shared mailbox appears. Wait a bit and then close and restart the new Outlook again. Or, add the shared box manually.
Select Mail from the navigation pane in new Outlook.
In the Folder pane, right-click your account name, and select Add shared folder or mailbox.
In the Add shared folder or mailbox window, type the name of the mailbox you want to access, for example, info@contoso.com.
OR
Open a shared mailbox in a separate browser window
Use this method if you want to view and manage the email for the shared mailbox in its own browser window.