If you have access to another mailbox but it isn't showing up in Outlook, you can manually add it by following these steps.
- Click File, the Account Settings drop-down menu, and choose Account Settings.
- Under the "E-mail" tab, choose your account, and then click Change.
- Click More Settings..., click the Advanced tab, and then click Add.
- Type the name of the shared mailbox, and then click OK.
- Click OK, Next, and Finish.