Of course, the bigger question is whether or not a corporation is a person. One could ask the United States Supreme Court (oops, that has already been done, and even the justices could not agree). Or, one could do what most Americans do when a librarian is not available: ask an "expert". All in all the question has proven perplexing to many.
However, theSuperior Court in Marin County, California http://www.loweringthebar.net/2013/01/no-you-cant-carpool.html did not have a problem in upholding a citation given to Jonathan Frieman for driving in the car pool lane with only a set of "corporate papers" beside him. For the purposes of section 21655.5 of the California Vehicle Code, a corporation was found not to be a person.
The Massachusetts Department of Transportation has posted information, including frequently asked questions, concerning the Massachusetts Highway High Occupancy Vehicle Program on their website. To date, it appears that no one has asked about corporate riders.