Sorry to bother you, but I don't understand something:
Why a"Guest" must be part of a "group owner"?
As a "guest" I can access any shared folder in the network if the properties of the folder permit the full access to Everyone. At least on Windows.
If I remember well, Davide said that a user who is not domain member, can access with full permissions a shared folder, as a guest, logged to that folder with any username and any password (wrong username and password). In Help pop-up, I think is written the same. Let's see:
Anyway, I will try to do what you said.