Use Multiselect instead of dropdown customer groups code in backend Magento

Question

I'm using a custom redirect extension from iPragmatech to redirect a customer to a different URL after logging in. This is based on customer groups.

I have asked the developer for help which he gave me but I'm not fully experienced with programming.

At the moment I can only select one customer group in a dropdown in the backend. I can change the dropdown to a Multiselect but I also need to customize the _CustomerGroup method. In Model/Observer/Customer.php I need to change this line:

if($this->_CustomerGroup())

This is the total code:

    /*method for Login Redirection*/
public function customerLogin(Varien_Event_Observer $observer)
{    

    if (Mage::helper('redirection')->isEnabled()){  
        $lasturl = Mage::getSingleton('core/session')->getLastUrl();
        if (strpos(Mage::helper('core/http')->getHttpReferer(), 'checkout') === false){
            if (! preg_match("#customer/account/create#", $lasturl) && Mage::helper('redirection')->isoptionEnabled('login_redirection')) {
                    if($this->_CustomerGroup()) {
                    $_session = $this->_getSession();
                    $_session->setBeforeAuthUrl(Mage::helper('redirection')->setRedirectOnLogin());

      }
    }
  }
}

Is there someone who can help me out with a piece of code and help me get into the right direction?

Br,

Steve