To enable the visibility of shared mailboxes in Workspace 365. You'll first need to make sure you'll have them availabe in Office 365/Exchange, when done do the following in Workspace 365:
1. Go Email settings
2. Click Done with the correct settings/data in place
3. When succesfull, the previous message will turn into a clickable button, saying: 'Search shared mailboxes'. Click it.
4. When available, you'll be presented with the shared mailboxes to either read or also 'send as' from your account when you check the corresponding checkboxes and save your settings.