Negative discount group on some custommers

[perfumbg]

Hello, Im working on a membership design for my site, I made one simple change that works pretty fine:
I modified functions_prices.php around line 26

    if ($specials->RecordCount() > 0) {
//      if ($product->fields['products_priced_by_attribute'] == 1) {
        $special_price = ($specials->fields['specials_new_products_price']); 

to

    if ($specials->RecordCount() > 0) {
//      if ($product->fields['products_priced_by_attribute'] == 1) {
        if ($_SESSION['customer_id']) {$special_price = $specials->fields['specials_new_products_price'];}
        else {$special_price = (($specials->fields['specials_new_products_price'])*1.05);} 

that way all custommers that have registration and are logged in can see prices normal and the customers that are not logged will see the prices +5%, also that makes the changes in the cart and on shipping total.

The problem is here: I want to have an profile (lets name it - orders by phone) from wich I can make order with 5% above the regular price that registed user pay, because this 5% are the income that goes for phone and employe expenses.

Is there a way to create an group discount -5% (not +5%!) so I can assign it to that profile "orders by phone"?

I assume that I have to give

$_SESSION['customer_id'] = id (of that profile) 

but I cant find the right way to do it.

Thank you