.NET Framework Class Library: System.DirectoryServices Namespace
http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemdirectoryservices.asp

How to poll for changes to the Active Directory by using Visual C# .NET
http://support.microsoft.com/default.aspx?id=326091

HOW TO: Add a User to the Local System by Using Directory Services and Visual C# .NET
http://support.microsoft.com/default.aspx?id=306273

HOW TO: Access ADSI Objects in Visual C# .NET
http://support.microsoft.com/default.aspx?id=315716

Using Active Directory In ASP.NET - Enumerate Active Directory Users
http://www.codeproject.com/aspnet/adsi2.asp

Querying Active Directory using .NET classes and LDAP queries
http://www.codeproject.com/dotnet/activedirquery.asp

How to get the list of groups that a user is a member of
http://www.codeproject.com/dotnet/usergroupmembership.asp