As of 2017 this no longer functions for Apple Devices. Android devices may still add shared mailboxes using manual setup.
Your main account requires full delegate access in order to add a shared mailbox.
1. Open 'Settings'.
2. Select 'Mail, Contacts, Calendars', and then select 'Add Account'.
3. Select 'Other'.
4. Select 'Add Mail Account' and fill in the details below as prompted.
5. You will need to manually enter the incoming and outgoing mail servers.
Host name: outlook.office365.com
Username will be in the following format: youremail@address\sharedmailboxalias
Password: your normal account's password
Email address: email@example.com
Shared account name: firstname.lastname@example.org
Password: password for email@example.com