1. From Office 365, click on the silhouette in the top right-hand corner 
  2. From the Office 365 menu, select "Open another mailbox"
  3. Office 365 Menu from Silhouette
  4. In the "Open another mailbox" dialog box, enter the name of the shared mailbox. If no name can be found in the directory, either the incorrect name was entered or the mailbox does not exist. Enter the name and click "Search Directory" to confirm the name of the shared mailbox.
  5. Open another mailbox - Office 365
  6. Once the correct mailbox appears in the box, Click "Open" Open Button - Office 365 and the shared mailbox will open in a new tab in your browser.

ATTACHED: A document with a guide for opening another user's mailbox in Office 365.