I've been on RCCl and Carnival and enjoyed both equally. IMO the food was slightly better on Carnival and I think (not sure) they still have 24 hour pizza which we loved.
As far and getting a group of people together, in the past we have planned almost a year ahead of time and gotten groups of anywhere from 25-50 people together to go on a crusie. You get group rates and certain perks. So the point I'm trying to make is that if you plan waaay ahead of time then people can plan for their vacations from work so they can go cruising.