NethServer Version: 7.3.1611 (Final)
Module: Shared Mailbox
Set up a shared mailbox under Management>Email Addresses>Shared Mailboxes.
Open Roundcube Mailweb client, go to Settings>Folders>(subscribe the folder by checking it).
After doing so, you can access the folder via Outlook, and Mozilla Thunderbird. (inside Public Folder)
On SOGo, click on the Shared Folder icon, it should open to reveal the shared mailbox.