Because this has always been ambiguous to me, this morning (in advance of a 2-week rental where we will be driving a lot), my wife accompanied me to ORD National so that she could present her license to the National counter in case that was needed to allow her to drive in case that was needed. The agent there said that was unnecessary and that she was automatically included (i.e., no need to input any driver's license info). As a double check, when I mentioned the same at the gate agent (when leaving the facility), I was given the same answer. Not sure that is legally binding, but that is the answer I was given this morning by two National employees!