linux - Execute a command as root

I'm trying to create a ruby script that is executed with root permissions when run by an unprivileged user. Basically I'm writing a wrapper script that when run adds the user to a group, runs a command, then removed the user from the group. This is all under CentOS and not using sudo.

I've played around with having the script owned by root and then chmod +s which as I understand should run the script with root permissions. However when I run the ruby command system "gpasswd -a #{user} #{group}" in my script I get a permission denied message.

I'm a bit stuck now no how to get this working.

Thanks