Setting default permissions for newly created files and sub-directories under a directory in Linux?

To get the right ownership, you can set the group setuid bit on the directory with

chmod g+rwxs dirname

This will ensure that files created in the directory are owned by the group. You should then make sure everyone runs with umask 002 or 007 or something of that nature—this is why Debian and many other linux systems are configured with per-user groups by default.

I don’t know of a way to force the permissions you want if the user’s umask is too strong.