There's a GroupUser WMI class but I've never tried to use it:
Win32_GroupUser Class (Windows)
I did find this post on CodeProject, which may be more than you need, but it wraps the WMI behavior inside C# classes. Even if you don't need/use the wrappers it should help with the WMI stuff:
Re: Users in local users and groups [modified] - C# Discussion Boards - CodeProject
You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.