When we used them we have always had good luck. They are the company we use most in the island. I am fairly sure we were not charged for an extra driver.
Their website shows this policy:
Authorized Drivers: The DOLLAR vehicle may be driven only by an authorized driver. An authorized driver is the renter and any additional person who appears at the time of rental and signs the rental agreement. All authorized drivers must satisfy our age requirements, have a valid drivers license, and fulfill our other qualifications, which vary by location. Employers and co-employees of renters renting on corporate rate plans, if properly licensed and meet our age requirements, are authorized to drive the vehicle while acting within the scope of their employment duties. See our credit requirements and drivers license requirements below. The authorized driver and any additional authorized drivers may be subject to a driver record check (See below). No other persons are authorized to drive the vehicle.
http://www.dollar.com/AboutUs/GeneralPolicies.aspx