No, could only use LDAP if those NT 4 systems were members of a 2000 AD

Still, not sure why you want to set a Primary Group. I realize the user must have one by default, and if they only have one, that will be the Primary. However, what application or method are you thinking of that requires your users to be in a specific Primary Group.

I mean you can add/move/modify/search/etc... regardless of what the Primary Group is, so maybe that is something I'm missing here.