http://www.codeproject.com/dotnet/QueryADwithDotNet.asp

Let's get a list of users belonging to a particular AD group. The code below
shows how to do this:

ArrayList GetADGroupUsers(string groupName)
{    SearchResult result;
   DirectorySearcher search = new DirectorySearcher();
   search.Filter = String.Format("(cn={0})", groupName);
   search.PropertiesToLoad.Add("member");
   result = search.FindOne();

   ArrayList userNames = new ArrayList();
   if (result != null)
   {
       for (int counter = 0; counter <
                result.Properties["member"].Count; counter++)
       {
           string user = (string)result.Properties["member"][counter];
           userNames.Add(user);
       }
   }
   return userNames;
}