Not really. The cruise line won't divulge this information, not even to travel agents.
You can try to google your ship name and sail date, and see if anything pops up.
More than likely you'll only find large affinity groups that way, because they'll advertise online. For example, google Monarch of the Seas May 2, 2008. In the first page of results, you don't find anything about Clay Aiken fans being on that cruise (and they will be).
Also, wedding groups and business groups might not show up at all, since those aren't always "advertised" widely online.