former egencia...
As a former egencia employee I would speculate, based on my coding and 'account setup' experience that the code used "is" a corporate code, specific to your employer. I would be shocked if it is a general expedia/egencia type code...shocked.
Specific to the corporate codes you would likely need to call National to get into the fine print and/or cozy up with your the person that manages your corporate travel contracts to define the 'extra driver rules'.
Each can be different however, it is rather common that corp.codes allow for extra driver at no-charge if they are co-workers and/or are immediate family with same residential address.
In short though, don't assume an immediate family member is covered - be certain - or assume they are NOT covered.