The next step on the mass user creation from my previous post was to add all those users into a group. Another dirty way for doing this was to get a list from /etc/passwd and change the format for the /etc/group file.
[root@hostname etc]$ cut -d: -f1 passwd | tr "\\n" ","
That makes the job. With the output you can just paste the string into the /etc/group file on the required row.