You want to set up a shared folder on the same Mac, like iTunes or Aperture, so your family can share the same files. Let’s assume you have a group called “staff”; which everybody is already in, and a folder called “common”. This is a mac, this should be simple, right? well.. almost with a bit of terminal-fu:

FOLDER=common
GROUP=staff
mkdir $FOLDER
chown -R $USER $FOLDER
chown -R :$GROUP $FOLDER
chmod -R g+rw $FOLDER
chmod -RN $FOLDER
chmod -RI $FOLDER
chmod -R +a '$GROUP allow list,add_file,search,delete,\
  add_subdirectory,delete_child,file_inherit,directory_inherit' \
  $FOLDER
ls -lde $FOLDER

You can use the slightly more permissive '$GROUP allow list,add_file,search, \ delete,add_subdirectory,delete_child,readattr,writeattr,readextattr, \ writeextattr,readsecurity,file_inherit,directory_inherit' which allows that group to change permissions and ACLs as well.

By explanation;

• chown -R :staff changes the standard unix group
• chmod -RN and -RI remove any inherited and initial ACLs; this is not needed for new folders but if you are converting an existing folder, this helps a lot to clean up crap
• chmod -R +a this is the permissions-mojo being applied
• ls -lde displays the extended permissions for a folder note that the ACLs must not have spaces in them if you are pasting code from above

Further reading:

• the manual
• sage uni
• ars technica
• ideo