Once the setting is finished the administrators can use AD groups and accounts for access privileges management. QNAP "File Station" applet prompts to use Windows ACL management (for example - "Security" property sheet of Windows Explorer) for access security control:
Unfortunately, the privileges set using Windows Explorer property sheet do not apply until top level access for AD users/groups for selected share is set (using "File Station" applet):
Once the access to the top level of file share is set at least to "Read only" - further access privileges management can be performed using Windows ACL tools (Windows Explorer, scripting etc.).
Enjoy!