Microsoft help doc I used and it has extra information about notifications Open and use a shared mailbox in Outlook Web App (microsoft.com) 


  1. First log into your office and go to outlook Outlook (office.com).
  2. Right click on the Folder and then click Add shared Folder.
  3.  
  4. Enter the Email or name of the shared mailbox then click add.
  5.    
  6. This will add the mailbox at the bottom of the sidebar like below.