There are three options for accessing shared mailboxes:
- Add mailbox to current account (No Send-As option)
- Add mailbox as second account (Send-As possible)
- Open Mailboxes through Office365 Web App (Send-As possible)
If you simply need to view the mailbox contents, option 1 is sufficient. Either 2 or 3 will allow you to send from that email address, but if you use a lot of different computers option 3 may be easier for you.