On a Windows PC? See this article.
If you're on a Mac, read on below.
To open a shared mailbox:
- Go to Outlook
- Choose the File Menu
- Select Open >> Shared Mailbox
- Search for and open your desired mailbox.
See Microsoft's full instructions here: https://support.office.com/en-us/article/open-a-shared-folder-or-mailbox-in-outlook-for-mac-6ecc39c5-5577-4a1d-b18c-bbdc92972cb2.