I was under the impression that you needed to limit access to key OP system settings in order keep a rogue user from messing up other important settings like the registry or the control panel. That is what the Group Policy Editor is for.
As far as file access goes, If the Users accounts are located on the local machine you should be able to just right click the file, then choose "share with", then choose "specific people" then right click the little down arrow and choose the User name you want to set permissions for. After that you can choose read or write or both read/write permissions for that particular file.
I'm not really sure what problems you might be having with it but it looks simple enough to me.