It happened to Spike Lee.  Indie FilmMakers Need to Protect Themselves!

Joe is a young filmmaker shooting his first movie, which is a fictionalized account of a historic event. Since Joe has virtually no budget, he gets friends, relatives, and others to assist and work for peanuts. Joe's college roommate, Ben, who is an expert on the subject of the film, helps prepare the actors for their roles. He suggests extensive script revisions to make the film more historically accurate, creates some scenes for the movie, and helps edit some parts of the movie in postproduction. 

Much to everyone's surprise the film becomes a blockbuster. Ben requests that he be given a writing credit, but Joe only agrees to credit him as a "technical consultant". Instead of getting mad, Ben applies for a copyright with the U.S. Copyright Office, claiming that he is the co-creator, co-writer, and co-director of the film.

Sounds like a mess, doesn't it? It certainly is unless you take some precautions from the beginning. First, anyone assisting you (like Ben) should have a written a contract, which clearly states their involvement including duties, credit, and ownership of the film, etc. Often, these are called "work for hire" agreements in which the person acknowledges that any thing they contribute in the form of copyrightable material is yours. 

Whether you are paying them actual money or points, you only need to make sure that they are given some consideration for their efforts (something of perceived value). Spend the money and have a lawyer draft this for you. If you do it yourself, you'll get what you pay for.

If it is too late for a written contract, the law may still be on your side. The scenario I gave above doesn't only happen in the Indie world, it even happens with the major players. Take the case of Aalmuhammed v. Lee,Warner Brothers, et. al., decided by the U.S. Ninth Circuit Court of Appeals in February 2000.

In 1991, Warner Brothers hired Spike Lee and his production companies to make the movie Malcolm X. Denzel Washington, who starred as Malcolm X in the picture, hired Aalmuhammed to assist him in preparing for the role. Aalmuhammed had previously written, directed, and produced a documentary film on Malcolm X. 

On the set, Aalmuhammed suggested extensive script revisions (some were included, some were not) to ensure the religious and historical accuracy and authenticity of scenes depicting Malcolm X's religious conversion and pilgrimage to Mecca. Aalmuhammed directed Washington and other actors on the set, created at least two entire scenes with new characters, translated Arabic into English for subtitles, supplied his own voice for voice-overs, selected proper prayers and religious practices for the characters, and edited parts of the movie during postproduction.