Stupid or not, regardless of whether you charge admission or it's a free event, you must obtain the rights to perform the piece.
If you're doing an event that doesn't charge admission (totally free, not a benefit...nobody makes money, no money collected whatsoever), and it's a one-night event, there's usually a discounted rate. All of this depends on the situation, the size of the venue, and lots of other technicalities that the producer of the event and the licensing agent (Dramatists, Samuel French, MTI, Tams-Witmark, R&H, etc.) work out prior to the start of rehearsals of any licensed piece.
The last thing anybody wants is a lawsuit, and that happens much more frequently than you might suppose -- pay the licensing expenses -- or look forward to some crazy expensive court costs (you could be sued by the playwright, the licensing agency, the original producers of the show, the playwrights agents/guilds, etc ad infinitum!) Don't expose yourself to that legal quagmire.
I'm sorry, but TooDarnHot is wrong on this one. Keep legit!
<---- You can see the crazy in her eyes. ;-)