Office 365 (Outlook 2010 or 2013) - Add a Shared Mailbox
Add another mailbox to your Outlook desktop client profile.
If you frequently work with a shared mailbox's account folders, you will want to add the shared Office 365 mailbox to your Outlook desktop profile. Doing so will automatically open the shared mailbox every time that you open Outlook. The shared mailbox appears in the Folder Pane underneath your Office 365 mailbox folders.
1. Open up the Outlook desktop client (instructions include images for Outlook 2013, but these instructions also work in Outlook 2010). With your email open, click the File tab, click Account Settings, and select Account Settings:
2. This will cause a window to pop up titled Account Settings. In the E-mail tab, click the proper exchange account name to highlight it, and then click the Change button.
3. This will cause a Change Account window to pop up. On the bottom right of that window click the More Settings button.
4. Another pop-up window will appear, this one entitled 'Microsoft Exchange'. Under the 'Advanced' tab, click the Add button.
5. Last pop-up - Type in the name of the mailbox, in my case it's 'firstname.lastname@example.org' and click OK.
6. The name of the mailbox you are adding will now be displayed in the 'Open these additional mailboxes:' box. Click Apply and then OK to close this window.
7. All that's left is to close out the remaining pop-up windows until only the main Outlook window is left. If you check out the navigation pane on the left side of your window, you should see the desired new mailbox.