How to add a shared mailbox in Outlook (Mac OS)

1. Open Outlook, select the Tools menu on top, then select Account.
2. Click on the + Symbol on the bottom of the accounts list
3. Type in the Shared mailbox Email Address that you want to add.
4. At the Enter Password Prompt, click on Sign In with Another Account.
5. Then login with your own Email and Password.
6. Click on Done.

As of Sept 2021, the New Outlook has a bit different method:

To open the shared mailbox:

1. In the Tools menu, choose Accounts, and select the account that has access to the mailbox.
2. Select Delegation and Sharing
3. Choose Shared With Me tab
4. Choose + to add a shared or delegated mailbox