How to replicate the behaviour of /usr/local

[Klistvud <quotations@aliceadsl.fr>]

Howdie, fellow Debianites!

I have a shared directory on my system; what I'd like to achieve is 
making every newly created (or copied from elsewhere) file belong to 
the group owner "users". In short, I have been trying to replicate the 
behaviour of the /usr/local directory. Let me explain: if I copy a file 
(any file from anywhere) with Nautilus or Gnome-Commander 
to /usr/local, the original file owner group gets changed to 
"staff" automagically. I'm trying to make my /home/shared dir behave 
the same way. I've tried playing with chmod and chown, I've even cloned 
the directory permissions (with 'chmod --reference=/usr/local') to my /
home/shared directory, but that didn't help. Is this behaviour 
implemented via file permissions at all, or via some system-wide 
setting or what?

TIA
-- 
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