The first step in this process is to be granted access to the Shared Mailbox. The owner of the mailbox will have to request access for you from firstname.lastname@example.org.
Once you have been granted access, you will need to add the account in Outlook. To do this, click on Outlook in the upper left corner of the screen, and select Preferences. Then go to Accounts. Click the plus button at the lower left of the window, and select Exchange from the popup menu.
In the E-mail address field, type the full email address of the shared mailbox; in this case, email@example.com. In the User name field, type IPGNA\first.last (first.last being YOUR user name, as you are using your account to authenticate access to this mailbox), and then type your password in the last field, and click the Add Account button.
Lastly, you will get a popup asking if you want to "Always use my response for this server". Check the box next to this, and click the Allow button.
Once the account is connected, it will most likely name the new mailbox Commonwealth-team 1 by default. This is just the name used to display this mailbox in your Outlook, so you change the name is Account Description to whatever you prefer. I've named this mailbox "Governance". This will only determine how the mailbox is labeled on your machine.
The new mailbox should appear below your default mailbox in the navigation pane on the left in Outlook.
To send a message from this shared mailbox, you'll notice that the From field in the new message window is now a drop down menu. Any shared accounts that you've joined will show up here to send from, simply select the one you want and the message will be sent from that mailbox.