Maybe try this way to put the syntax as :
...
...
<group>
<id>package_services</id>
<name>Services Server Packages</name>
<default>false</default>
<uservisible>true</uservisible>
<packagelist>
<packagereq type="default">proftpd</packagereq>
<packagereq type="default">httpd</packagereq>
<packagereq type="default">bind</packagereq>
</packagelist>
</group>
...
...
...
<grouphierarchy>
<category>
<name>Services</name>
<subcategories>
<subcategory>package_services</subcategory>
<subcategory>package_services_2</subcategory>
<subcategory>package_services_3</subcategory>
</subcategories>
</category>
</grouphierarchy>
</comps>
Don't put :
<grouplist>
<groupreq>base</groupreq>
</grouplist>
in
<id>my_group</id>
Of course you should have the core and base description in the comps.xml
but this is the way to select packages
manualy. Try to have a real look to the original comps.xml and you'll
see much more how it runs. This works well for me.
Hope this help you
Vincent
