We have not cruised with a group, but have often seen other large groups. None so large as to dominate the cruise. Once we sailed with a large group of pax with MS - many in wheelchairs or using walkers of some sort. This cause some congestion in the Lido buffet but it made me smile that they were able to travel.
Our last cruise there was a large group of 100+ that was essentially virtual. They had banded together to get a group rate. I forget they name of the group, something like Happy Rovers, but they have apparently been around a while and travel once a year.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014