Generally it is best to book directly or through a local travel agent. There are no real advantages to online travel sites in the cruise industry because any travel agent can give you the same deal the cruise line themselves can. Cruise lines depend almost wholly on relationships with travel agents to book staterooms and local agents can also be a great source of advice, where places like expedia.com will leave you hanging out there on your own. If you are going to use an online travel site aside from the cruise lines themselves, go with one like cruisedirect.com where you can call and get advice from a real travel agent.
Feel free to contact the one that does a good deal of our Cruisemates group cruises, just look in the group cruise section. Its always a good idea to compare prices, some have group space where you can book and save some $.