This is squarely a legal issue so I strongly recommend that you consult with your organization's legal counsel, especially since FAR 1.602-3 requires legal counsel approval of ratification for any unauthorized commitment. That being said, FAR 1.602-3(a) defines an unauthorized commitment as "...an agreement that is not binding solely because the Government representative who made it lacked the authority to enter into that agreement on behalf of the Government." Note that the term "Government representative" is used rather than "Government employee." This could conceivably lead to the conclusion that a volunteer who has a relationship working for a government agency could be viewed as a "Government representative"...even if he/she is not actually a paid employee. Again, this could be splitting legal hairs so consult legal counsel.