A quote from the US patent and trademark office (http://www.uspto.gov/web/offices/dco...efresher.htm):
Originally Posted by spoolman
"For works created by individual authors on or after January 1, 1978, copyright protection begins at the moment of creation and lasts for a period of 70 years after the author's death. In the case of "a joint work” (prepared by two or more authors) the term lasts for 70 years after the last surviving author's death. "
What is unclear to me is the meaning of "at the moment of creation". Does that mean at the moment the exposure was made? Probably not, since the creative work is not completed at that point, at least not with print film. Does it mean at the moment of development of the negatives? Does it mean when the print was made? I don't know, but these seem to be important questions.
It seems to me that if you develop and print the film then it is a "joint work", and you would have equal interest to the work with the person who snapped the shutter. Under US patent law (which may or may not apply to copyright law, I don't know) unless there is an agreement of other provisions to the contrary, all inventors have 100% undivided interest in the work, which means that each can market or use the work without permission from the others. If copyright law works the same way, and if developing and printing the photos counts as part of the creation, making it a joint work, then it would seem to me that you would be free to sell the work without permission from the persons who snapped the photos.
I am not a lawyer, so take my comments with a grain of salt.