Sailing on group cruise is normally LOTS of fun... but they don't just happen. There can be a considerable amount of work involved.
First off, you should be working with a travel agent who has alot of group cruise experience, rather than directly with the cruise lines. An agent can negotiate a lot of value added bonuses for you.
Everything from cabin upgrades, complimentary cocktail parties, group photos, shipboard credits, free champagne or wine etc., can be negotiated, particularily if you're flexible, and can choose some of the less popular sailing dates.
The norm for groups is one free berth for every 8 cabins booked (the cost of prt chges and taxes has to paid), but on some sailing that the cruise lines may want to push, this could be reduced to one free berth for every 5 cabins booked.
Once you get a chose a ship and get a deal in place, it helps to increase the fun, planning some group activities onboard. Of course, no one is obligated to attend, but we've also found people in the groups appreciate the social gatherings, and even some group excursions.