Lots of organizations, lots of crews both paid and volunteer, as Cruddy Toes said. Add the DOC with both paid and unpaid crews to the list. The biggies in the Whites are the AMC, RMC, and USFS, who do a big share of the heavy lifting. The WODC, DOC, WV, and others all maintain a good sized chunk of trail as well. There are ways to volunteer with pretty much all of the organizations as well, plus organized volunteer crews.
-dave-