It's possible, but there are conditions and you'll have to work for it.
If you can gather a group of 10 to 16 people who want to go on a particular
cruise, you may be able to go for free as the tour conductor (this is
sometimes referred to in the travel industry as a "Pied Piper").
We strongly recommend that you talk to your travel agent
before you start gathering people, as tour conductor berths are not offered on
every sailing, and the number of people you'll need to get varies by cruise
line and other conditions.
A couple other things to bear in mind. The berth is free, but you still
may be responsible for other charges, like port taxes, tips, booze, shore
excursions, etc. As the tour conductor for your group, you'll be the
liaison between the group and your travel agent during the "get your paperwork in
order" phase before the cruise, and the liaison between the group and the
cruise line personnel once the cruise is under way.