A newly created file will always be owned by the user that created it, and (by default) the group it's a member of.
If, however, you set the
setgid access right flag on the parent directory, new files and subdirectories created under it will inherit the parent directory's groupID instead.
~$ sudo addgroup testing
Adding group `testing' (GID 1001) ...
~$ mkdir dir
~$ chown :testing dir/ # Change owner to group "testing"
~$ chmod g+s dir/ # Set the setgid flag
~$ ls -ld dir/
drwxr-sr-x 2 root testing 4096 2010-10-19 04:20 dir/
If you now create (as root) a new file under the dir/ directory, it will be owned by user root and group testing instead of user root and group root:
~$ touch dir/file
~$ ls -l dir/file
-rw-r--r-- 1 root testing 0 2010-10-19 04:46 dir/file
You will probably want to look at the umask command as well.