Dougy Mak's Blog

How Unix works is, when a user creates a file, the file’s group is the user’s, which means other users could not edit it. So my developers and I always had this problem, and I always had to use “chown -R www-data:www-data . ” to change all the groups of the files in the current directory, but there’s a better way. You can set up setuid.

$ chgrp ftpusers /path/to/the/ftp/directory
$ chmod g+s $_

More information: http://www.cyberciti.biz/faq/unix-bsd-linux-setuid-file/