Go through an agency. They can set up a group space and possible reduction on cabin prices.
You would be the group leader to be kept informed.
The other item is that so many cabins booked will earn Tour Conductor Points (TC). The TC can offer a free cabin depending how many cabins are booked or use the TC for social events, as cocktail parties, group pictures and not elect to get the free cabin. Normally it takes 16 cabins booked to get one cabin free.
The TA can handle the whole booking and take it off your shoulders. Each couple booking then can call the TA book under the group number and away you go.
A good TA will explain how the TC points work.