When logged you you will need to access your email - click on the Outlook icon on the left hand side menu:
Once you have opened Outlook in your browser click on your name in the top right corner, then select 'Open another mailbox'
In the box that appears, type the name or email of the mailbox you wish to open
When you have the correct one, click on it in the list and it will move to the box then click Open
Your shared mailbox will then open in a new tab or window in your browser and you can use the shared mailbox like a stand alone normal mailbox. You can also bookmark the shared mailbox for easy access at a later date.