If you really need XenForo integrated in your website, you could build an entire product like xfSite does, or just add these lines (which I use on a production website) "globally":
[PHP]require_once(XF_ROOT . '/library/XenForo/Autoloader.php');
XenForo_Autoloader::getInstance()->setupAutoloader(XF_ROOT . '/library');
XenForo_Application::initialize(XF_ROOT . '/library', XF_ROOT);
XenForo_Application::set('page_start_time', TIMENOW);
XenForo_Application::disablePhpErrorHandler();
XenForo_Application::setDebugMode(false);
if (!SESSION_BYPASS)
{//when user info and sessions are needed
XenForo_Session::startPublicSession();
$visitor = XenForo_Visitor::getInstance();
if ($visitor->getUserId())
{
$userModel = XenForo_Model::create('XenForo_Model_User');
$userinfo = $userModel->getFullUserById($visitor->getUserId());
}
}[/PHP]