I have only done one group cruise and it had a little over 100 cruisers in it. We all boarded in the normal fashion. If you had VIP status you went through the priority boardin, if not, you stood in the regular line. We then all had a Meet and Greet on board later that day.
One thing about Priority Boarding is that often it is slower than the standard line. When we first had priority boarding, we could choose to get into the regular line if it was shorter, but not anymore. We have actually been the last of 5 families to board by using the Priority boarding and we all arrived at the terminal together.
With the increase in passengers who have 10 or more cruises, the VIP lounge should have more than 1 or 2 boarding agents assisting us to make this go more quickly.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days