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.