Setting permissions

Um, that's kinda the point of having separate groups -- to keep folks that shouldn't be doing so from changing another's files.  For each folder, there is a setting for "Default access", and it sounds like you want to check the box that says Specify permission when new file/folder is created, otherwise inherit permission from parent.  But you are probably going to have to clean up some existing permissions as well.  You can do that from the NAS GUI, but it is often easier to use Windows to do so (assuming you have only Windows users).  Right-click on the folder in Windows, then select Properties, then Security, then Advanced.