First off try to find out how many people may go for sure and which ship, location and dates that are acceptable to you. Also try to figure out what type category the people expect. Keep in mind that you can have several options it does not have to be narrowed down to one option.
One thing you should know is that a group cruise is NOT always cheaper than booking everyone on an individual basis. What the group cruise will do is give you some options for possible perks such as a private get together or a free berth if there are enough people. If you go with the group cruise DISCUSS what they will try to get for you as extras even though they will not fully know what is available until the know the exact number of bookings and final payments have been made.
Approach a TA and give them all your preliminary information and also let them know if you want the group opened to others that may book with you but are not sure if they will book with you. Give the TA a little time to do some research for the best deal. If you want to open it to others the TA can get information and flyers for you to provide to others to help you entice a few more people.
Make sure you and the TA fully understand what is expected by both of you and who will be the main contact.