//Authenticate a User Against the Directory
        private bool Authenticate(string userName,string password, string domain)
        {

            if (userName == "" || password == "")
            {
                return false;
            }

            bool authentic = false;
            try
            {
                DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain,userName, password);
                object nativeObject = entry.NativeObject;
                authentic = true;
            }
            catch (DirectoryServicesCOMException) { }
            return authentic;
        }

private bool Authenticate(string userName,string password, string domain, string group)

bool IsInGroup(string user, string group)
{
    using (var identity = new WindowsIdentity(user))
    {
        var principal = new WindowsPrincipal(identity);
        return principal.IsInRole(group);
    }
}

public bool AuthenticateGroup(string userName, string password, string domain, string group)
    {


        if (userName == "" || password == "")
        {
            return false;
        }

        try
        {
            DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, userName, password);
            DirectorySearcher mySearcher = new DirectorySearcher(entry);
            mySearcher.Filter = "(&(objectClass=user)(|(cn=" + userName + ")(sAMAccountName=" + userName + ")))";
            SearchResult result = mySearcher.FindOne();

            foreach (string GroupPath in result.Properties["memberOf"])
            {
                if (GroupPath.Contains(group))
                {
                    return true;
                }
            }
        }
        catch (DirectoryServicesCOMException)
        {
        }
        return false;
    }