Actually, the number of viewers has nothing to do with it: http://www.mtholyoke.edu/offices/student-programs/Rules_and_Tools/movies.on.campus.shtmlWelcome to the Shack Jeff,
I would be very careful about this as showing a movie to more that 12 people is a violation of copy write laws (paying or non paying it doesn't matter). Its one thing to show 15 or 20 of your friends in your home but outside is another ballgame.
The Motion picture industry would have you believe that simply showing the movie outside your home requires a special license, or that there is some magic number above which a private screening automatically becomes a public screening. In reality, even if the true law (private being a group of friends or common acquaintances) is on your side, if the license holders decide to sue you, you'll be better off paying the multi-thousand dollar settlement instead of the lawyer fees.
Trust me. I work at a college, we've been sued, I know the feeling.