Besides having repositories defined, are they enabled and set to refresh? Is the subscription current, assuming there is one?
Since RHEL 7.0 is now out, are the 6.0 repositories even still available?
If you know what desktop you plan to run, you might do a search for the a package, (ie. yum search kdeworkspace) and see what you get. If it is there, you could install it and YUM should add whatever packages are needed to satisfy prerequisites. Be prepared for a rather large download.
I found two pages concerning YUM commands: http://yum.baseurl.org/wiki/YumCommands
. The latter is specifically for commands concerning groups. Good luck.