var searchRoot = new DirectoryEntry(@"LDAP://DC=subdom,DC=ourdomain,DC=com", @"domain\iis-appPool-username", "password");
searchRoot.AuthenticationType = AuthenticationTypes.ReadonlyServer;
using (var searcher = new DirectorySearcher(searchRoot)) // Specify the search root here
{
searcher.Filter = string.Format("(&(objectClass=group)(sAMAccountName={0}))", "someGroupName");
searcher.PropertiesToLoad.Add("distinguishedName");
var result = searcher.FindOne();
}