// Where all visitors can access
$allowed_routes = array(
    'product/category',
    'common/home',
    'account/register',
    'account/login',
    'account/logout',
    'account/forgotten',
);

// Who can access
$allowed_customer_group_id = array(
    1,
    2,
    3
);

$current_customer_group_id = $this->config->get('config_customer_group_id');
if($this->request->get['route']){
    $current_page = $this->request->get['route'];
} else {
    $current_page = 'common/home';
}

// notification
$data['notice'] = '';

if(!in_array($current_customer_group_id, $allowed_customer_group_id) || !$this->customer->isLogged()){
    if(!in_array($current_page, $allowed_routes)){
        // not allowed
        // do something, redirect to login page
        if($this->customer->isLogged()){
            $this->response->redirect($this->url->link('common/home', '', true));
        } else {
            $this->response->redirect($this->url->link('account/login', '', true));
            // or $this->response->redirect($this->url->link('account/register', '', true));
        }
    } else {
        $data['notice'] = 'Some notification message here!';
    }
}

{% if notice %}
    <p class="alert alert-info">{{ notice }}</p>
{% endif %}