System groups join servers and channels in having their own IDs. This value, the sgid, is a required parameter for the set_group_membership method, for instance. Like the sid and cid, the sgid can be obtained through the RHN website. Just click on the name of a system group and view the end of the URL, something like, "details.pxt?sgid=334958". Note that the member parameter within the set_group_membership method requires only yes or no as input to make the association.