Permissions for new files in shared directory
You can't do this with traditional POSIX-style permissions, but you can with inheritable access control entries.  To allow read+write access for the entire "staff" group to everything in /Users/Shared/reallyshared, you'd use:

sudo chmod -R +a "staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /Users/Shared/reallyshared


The file_inherit,directory_inherit part means that this access control entry will automatically be added to new files &amp; folders created inside this folder, but it doesn't automatically apply to items already there (that's why I added the -R flag to chmod -- that applies it to everything currently within the folder), and it won't be applied to items created somewhere else and then moved into this folder (I don't know of a way around this, sorry).