Creating completely custom Package and repodata

Hello,

I am completely new to this, so this a totally newbie question.  I have 
been asked to create a very custom CentOS 6.4 release for internal use.  
I have had no problems with creating a kickstart file and customizing it 
from the options in "http://fedoraproject.org/wiki/Anaconda/Kickstart", 
and all my %pre scripts have worked great.  The only place i am stuck at 
is creating a a completely a custom "core" group and other custom 
groups.  My only attempt so far was to create a custom group by:

yum-groups-manager --merge repodata/<*.comp.xml file>  -n "customgroup" 
--id=customgroup --description "Custom packages" -c yum.conf 
--save=repodata/<*.comp.xml file> <package name>

(yum.conf is the repo for repo with the custom rpms) . When i added 
"customgroup" to the "%packages" section, as getting errors saying that 
the group can not be found.

before i run "createrepo" and try doing this all from scratch, i was 
hoping to at least get a custom group working with the existing repodata.

I might have completely missed it, but i can not find any detailed 
information on how the repodata and the files in there work, and how to 
setup a completely custom repo on the ISO.  Does anyone know where i can 
find detailed documentation explaining how this works, or point me to a 
tutorial on how to do what i am looking to do?  I not only need to get 
this done, but i also want to understand what it is doing and how it 
works so i can more self-sufficient and do all of these cleanly and 
correctly.

thank you,
atg