People with questions about all this DSC and MMSI stuff can check the following USCG/BOATUS sites for info (this is actually the BOATUS site that one is referred to by the USCG's site):
http://www.boatus.com/mmsi/faq.htm
Check out the navcen link within the list of items when you get into the BOATUS page.
See if you can figure out how a recreational boater would go about converting his MMSI into a GROUP MMSI number.