If XenServer's repositories don't support groups, you'll have to mirror the RPMs locally and merge your own group file into the metadata. Theoretically.
You could copy CentOS 6.2's group file.. or 5.6... I'm not sure which would be correct in this case. Either way, the filename ends in "comps.xml
" and is found in the repodata directory on the installation media. You can re-create the repodata with the group data with the following command;
createrepo -g /path/to/mirror/repodata/3a2723269...-c6-x86_64-comps.xml -pd /path/to/mirror
It goes without saying that what you are doing is unsupported. You might break things or it might not work at all. This is how groups work with Yum, though. Good luck.