rcscott44,
To target a single user, you can indeed use the Environment Variable condition as EricNichols suggested:
As for performing a "live" LDAP query rather than asking Windows whether the logged-on user is a member of a particular group: I'm afraid there are too many corner cases to deal with. The "is this user a direct member of this group" query is pretty simple, but nested groups, primary group support, referral chasing, foreign security principals, et cetera make it quite hard to mimic Windows' membership logic.
Ray_handels: Is such a "live" lookup what you were looking for, or did I misunderstand?