On RHEL6, it's possible to create groups using the group kickstart option.
However, this option is not documented nor described on the official documentation that can be found here : https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Installation_Guide/s1-kickstart2-options.html
It should be nice if the group option would be documented in order to avoid that new users into the kickstart world think that it is not possible to create groups.
Thank you for the report, Olivier. I'll take a look; it should be an easy fix.