There are two different MGB Registers that exist in North Amderica.
The American MGB Association (AMGBA), which historically holds combo-meets with existing British car shows throughout the States, is the club that is referred to in this thread. It was formed in the early 70's in Chicago by Steve Glowkowski, et al. I'm not sure how many additional cars they draw to the existing event that they coordinate with each year. The AMGBA is a privately owned club, similar to the MG Owners Club in England.
The North American MGB Register (NAMGBR) was formed in 1990 as an alternative to the AMGBA and is a Not-For-Profit organization, run by its members.
NAMGBR rotates its convention throughout the United States and Canada...the limiting factor that of needing a host club to assist in the organization of the event. NAMGBR meets, unlike AMGBA meets, are NEVER held in conjuction with an existing local show.
NAMGBR meets have been held in:
1992 - Peterborough, Ontario, Canada
1993 - St.Louis, MO
1994 - Washington, D.C.
1995 - North Lake Tahoe, NV/CA
1996 - Indianapolis, IN (the first of the all-M.G. Register gatherings)
1997 - Buffalo, NY
1998 - Hagerstown, MD
1999 - Vancouver, B.C., Canada
2000 - Cleveland, OH
2001 - St.Paul, MN (the second of the all-M.G. Register gatherings)
2002 - Grapevine, TX
2003 - St.Louis, MO
2004 - Parsippany, NJ
2005 - Olympia, WA
2006 - Gatlinburg, TN (the third of the all-M.G. Register gatherings)
2007 - Sonoma, CA
2008 - Valley Forge, PA
2009 - Breckenridge, CO
2010 - Belleville, Ontario, Canada
2011 - Reno, NV (the fourth of the all-M.G. Register gatherings)
I believe that bids are currently out for locations of 2012 and 2013.
I've driven/attended all of the above meets (with an MGB or MGC)except 2007, and I will be unable to make this year's event in Belleville.
The road trips to these events are just as enjoyable/memorable as the meets themselves!