you can do this using magento `event observer,create an event on payment_method_is_active` and depednds on customer group disable and enable payment method:

check this link:
http://magento.stackexchange.com/questions/24447/implementing-payment-method-per-currency/24502#

 and you need `do  on some change in observer.php`
I have trying to example  and try to modify according to  you

       public function filterpaymentmethod(Varien_Event_Observer $observer) {
    
    
            /* call get payment method */
            $method = $observer->getEvent()->getMethodInstance();
    if(Mage::getSingleton('customer/session')->isLoggedIn()) {
    
     $roleId = Mage::getSingleton('customer/session')->getCustomerGroupId();
     $role = Mage::getSingleton('customer/group')->load($roleId)->getData('customer_group_code');
    
          if($method->getCode()=='purchaseorder'){
                $quote = $observer->getEvent()->getQuote();
    
                if($role=$yourcustomergroupid){
                $result = $observer->getEvent()->getResult();   
                $result->isAvailable = true;
                return;
                }else{
                $result = $observer->getEvent()->getResult();   
                $result->isAvailable = false;
    	 }
            }
    
         if($method->getCode()=='checkmo'){
                $quote = $observer->getEvent()->getQuote();
    
                if($role=$yourcustomergroupid){
                $result = $observer->getEvent()->getResult();   
                $result->isAvailable = true;
                return;
                }else{
                $result = $observer->getEvent()->getResult();   
                $result->isAvailable = false;
    	 }
            }
    }
    
    }
**Note: If you not getting customer group id from session then you need  tp load  customer by customer id(which is must get from session)** 
 for getting customer group from  session try below:


http://stackoverflow.com/questions/9242390/showing-which-group-a-customer-belongs-to-in-magento
http://xhtmlandcsshelp.blogspot.in/2010/12/get-customer-group-id-in-magento.html