Set and keep owner / group for shared files and folders?

[fortissimo]

I've got 2 QNAP's running, TS-212P and TS-231P.
The drives are being shared to Linux and Windows computers.

Linux uses users and groups for permissions, Windows doesn't.
The users on Linux are in the same group so they can get to the files.
But, whenever Windows writes to the NAS it sets the rights to root:root.
This means that the Linux-user has to change the ownership before he can use the files.

Under Control Panel - Network/fileservices - Win/Max/NFS the workgroup has been set tot the correct group.
Where/how can I set that the Windows-devices uses the same group-name?