The best advice I can offer is for you to get together with your Cruise Specialist, especially one that also specializes in groups. There are alot options available to you - too many to go over in an email or a forum post. And it's extremely important to find the right cruise to fit your requirements, lifestyle, and budget. You need to decide which cruise line, what ship, and what itinerary. Plus, you have several options concerning where they'd like to get married. For example, they can get married on the ship at the embarkation port so people who want to attend the wedding but not go on the cruise can do so. They can get married on the ship while at sea. Or they can get married in a port at an exotic location.
Having been involved with many of these for our clients, I can tell you firsthand there a huge number of details that go into such a gathering and you can become overwhelmed very quickly. Then there's all the numerous questions the guests are going to have and they need someone to help them - needlessly to say, you don't want to be the middle person in all of that. So it's imperative to find a Cruise Specialist to help you. Their services can be invaluable in situations like this. It's their job to take the hassle out of all of it and there is no charge for their services, so it makes sense to let them do the work so you don't have to.