Hi Joanne,
If you got an exception, that would seem to indicate that you are using .NET/ASP.NET to build your web app. In that case, you should not be programming with the legacy COM objects used in classic ASP for this. You should be using System.DirectoryServices or System.DirectoryServices.AccountManagement.
Ryan and I wrote a whole book about this which is what the website was originally created to support. I don't want to say you have to buy it, but it might be really helpful to you and save you a lot of time.
However, it isn't really for novice .NET developers either. If you don't have any .NET skills or know ASP.NET, you might want to try to build up some of the basics first. Otherwise, you'll have a hard time. Such is the thing with learning any kind of new broad technology.
Best of luck!