Hahah, it's really no big deal. I don't THINK I'm giving any spoilers, especially if you've seen the movie, but... no guarantees.
At the beginning of the show you're given a glowstick, and for the last number ("Xanadu") and curtain calls, the actors (who break the fourth wall with you fairly often, but never in a singling-out or too abrasive way) tell you to crack the glowsticks, stand up, and dance where you are. The usher who leads you onstage at the beginning of the show tells you about the dance and "auditions" you. Basically they just want you to faux-lasso with your glowstick in hand, but of course you can do whatever you want (while standing in place)
It's a complete no-pressure thing, especially since... well, not to imply that any specific group of people is bad at dancing, but the majority of the onstage people when I was there were middle-aged or older gay men or Jewish grandparents, so you almost definitely won't be the most awkward person there!
"Why, I make more money than... than... than Calvin Coolidge! PUT TOGETHER!"
~Lina Lamont
My name wasn't, isn't, and will never be Scott.