"but if I don't know them, it makes me worry."
Worry seems to be one of my biggest challenges. I've found when I calm down and think all business, my horse calms down, too. The more shows I've gone to, the calmer I've gotten. It has a lot to do with experience. It also has to do with trusting your horse. If you do, that's the biggest hurdle right there!
Don't let all the other stuff MAKE you worry. Think of a postive thought, and the minute you start to worry, MAKE yourself use the positive thought. I have used "this is just business", "this pony can do this", "I'm just out for a nice ride, now worries." Things like that.
I would also ask your teacher when group lessons are so you can do ALL group lessons between now and the show. YOu really need the experience with other saround so you can learn how to make yourself think positive and tune the others out. Except, of course, you do need to know where the other riders are and what they're doing so you can avoid them.