>Is there a function in ADSI (or anywhere else) to ADD a new NT global group?? I need to be able to add a bunch of groups as a one-off batch job, so that I can control access to a bespoke system through the NT user manager.
>

oComputer = GETOBJECT("WinNT://ERIKMOORE,Computer")
oGroup = oComputer.Create("group", "SomeNewGroup")
oGroup.Description = "This is a new group created by ADSI"
oGroup.SetInfo()

This stuff is all documented in MSDN.

>BTW, I have ADSI on my NT4 SP6 machine, but what can I install on my customers' machines to ensure that they have it too?

There is a distributable file somewhere on the MSDN site that installs ADSI on pre Win2K Windows boxes. You'll have to search for it...