Increase Product Quantity based on attendees added Drupal Commerce
If you want to do this solely via Rules, then you'd likely need to write a custom Rules action that can count those attendees and apply an appropriate price adjustment to the unit price of the line item. If you want to bypass Rules completely, you can change your price calculation process to support module based alterations and write custom code that hooks into the sell price calculation process via hook_commerce_product_calculate_sell_price(). The challenge here is that you aren't getting these attendees until the checkout form, so you'll want to make sure the customer has a chance to review the final price before paying.