Follow these instructions to begin adding a new user to a shared inbox.
Please note that you will need to submit a helpdesk ticket for full access to the inbox. See below for details.
In Outlook, click File > Account Settings > Account Settings... :
In this window, double-click your account, or you can click Change...
Click More Settings:
Click the Advanced tab, then click Add...:
Type the name of the desired inbox and click OK:
In the settings window after you've added the new inbox, check to make sure that Download shared folders is checked, then click Apply and OK:
The shared mailbox will then appear below your other mailboxes. If the mailbox has a lot of contents, it may take some time before it downloads everything.