If you have full access permission to a shared mailbox that appears in the address book, you can use Outlook Web App or a desktop version of Outlook to open that mailbox.
Add a shared mailbox so it displays under your primary mailbox
If you want to monitor the email from your primary mailbox and the shared mailbox at the same time, use this method. In addition, after you complete this task, the shared mailbox and its folders are displayed in the left navigation pane each time you open Outlook Web App.
Sign in to your account in Outlook Web App.
Right-click your primary mailbox (where it says "Folder") in the left navigation pane, and then choose Add shared folder. (Your name is on your primary mailbox.)
In the Add shared folder dialog box, type the name or email address of the shared mailbox, and then select Add.
The shared mailbox displays in your Folder list in Outlook Web App. You can expand or collapse the shared mailbox folders as you can with your primary mailbox. You also can remove the shared mailbox from your Folder list. To remove it, right-click the shared mailbox, and then select Remove shared folder.