Step 1: Go to outlook.office.com


Step 2: Right-Click on the 'Folders' inbox

Step 3: Click 'Add shared folder'



Step 4: Type in the email address of the mailbox you want to add



Step 5: Click on the correct mailbox you want to add



Step 6: Click 'Add'



Step 7: You will now see your shared mailbox on the left hand side