For it to work at all, each user would have to have a different URL for each group they were in.
Along with nearly every other SSO solution short of LDAP, OpenID infuriatingly only authenticates the username — it provides no means whatsoever for context about the user, much less group membership.
Brad Fitzpatrick (the creator of OpenID and founder of LiveJournal) has been working on a new protocol called WebFinger to add context to usernames, but so far they've avoided trying to codify a mechanism for group membership.
