I met a new friend on a Halloween singles cruise last year. She mentioned that she was on a previous Halloween cruise organized by an internet dating service. That particular cruise had no organized activities, and the group coordinator could be found most times bellying up to one of the bars.
Sometimes, travel agents will simply book a group and just market that particular cruise as a "special" while other agents will arrange private cocktail parties and other fun goodies. This is a fairly common practice in the cruise industry. You do need to be careful when booking a group cruise. Make sure that the people you book with have a good track record of providing a quality expierence for their clients.