[CentOS] EL7 mirror: "There is no installed groups file."

I installed the RHEL 7 beta here to test while waiting for CentOS 7 to 
arrive.  On noticing that yum didn't work, I decided to set up a local 
mirror.  I rsync'd

   ftp://ftp.redhat.com/pub/redhat/rhel/beta/7/x86_64/os/Packages/

to a local web server here, then regenerated the repodata directory with 
createrepo.

Now yum works fine, for the most part.  "yum search foo" pulls up a 
plausible list of packages, "yum install bar" chases dependencies as 
expected, etc.

Unfortunately, "yum groupinstall" isn't working, which means I have no 
easy way to install Gnome on my minimal EL7 installation.  Apparently I 
need some kind of "groups file" to feed to createrepo --groupfile, but I 
don't know where to get one, or how to construct one.  I've dug around 
on ftp.redhat.com and can't find anything that looks plausible.

I've tried manually installing packages to build up this GNOME desktop, 
but despite installing dozens of things, startx still doesn't give me 
something usable.

I know I could get a GNOME desktop by reinstalling the OS, but that 
would wipe out a lot of the local work I've done on this VM so far.

The only reason I need X in the first place is that 
system-config-printer no longer runs in text mode.

(I'm trying to set up a CUPS server.  So yeah, X11 is a prerequisite for 
installing a printer now.  Lovely.)