Rick, the suggestion that just one person be in charge of making the arrangements is a good one. Whoever that is, make sure they're working with a travel agent who is familiar with booking groups, and familiar with cruise line procedures and policies. Discuss with the agent in detail exactly which types of rooms are needed, how many rooms you think you'll need including how many rooms for triples and quads, if any. Have the agent go over the payment schedule, when deposits are due for each room, and when final payment is due. The agent should also details the "perks" the group gets.
Also, make sure the group is clear about cancellation insurance, and other things the cruise price does not include, such as drinks, tips, transportation to the port (if that's not included in the price) and shore excursions.
As far as the group itself, most groups will do their own thing during the day. It's always a nice idea to reserve meeting space for a get-together or cocktail party the first day of the cruise, so everyone can meet and greet each other to start off the cruise. Some groups do this for the hour before dinner the first night, some do it right before the ship sails. It's totally up to you whether you formally reserve meeting space for other time during the cruise. Some groups like to do that towards the end of the cruise, to discuss where and when the next family reunion will be.
The cruise line tends to keep the group at adjacent tables in the dining room. That should be no problem. Doing it "round robin," as Susan suggested, is a good idea so that people can visit with everyone at dinner by the time the cruise is over. Some cruise lines stipulate that the group pre-pays the gratuities in order to be able to rotate where folks sit.
It's a good idea to have a "central information" area for the family, be it a web site someone sets up, or a Facebook page, or similar. That way, all the information is there, including prices, itinerary, etc, basically as much info as someone is willing to put on the site. It saves the group leader from tons of calls with folks asking the same questions. It's a good way to disburse information about the cruise, about payment due dates, etc.
I'll think of other things that I'll post at another time. Your family will have a great time!