Mac OS X must use strict rules for the standard permissions set on new files or folders, ensuring that you can use the system in environments with non-cooperative users. TinkerTool System can access the permission filter that controls what permissions applications are not allowed to grant when initially creating new file system objects. To allow applications run on the graphical user interface to grant write permission for the group owner, perform the following steps:
The new setting does not guarantee that applications will grant group write permission for objects they create. But they will be allowed to do that, after the setting has been established. Most applications, like the Finder, will indeed set group write permission for new objects afterwards.
Further information: The Pane System.