Sara, it sounds like you're already doing very well. Keep doing what you're doing, and I think you'll be fine at the ride. When I first started riding, I rode alone for several months--first mostly on bike paths and then on the road--before trying a club ride. I was really nervous about it, and in retrospect, I shouldn't have been quite that relunctant.
If you do decide to switch to clipless pedals, I would recommend spending some time practicing with them before hitting the road. When I first got mine, I propped myself and the bike in a doorway in my house and practicing getting in and out of one pedal at a time. It helped me get a feel for how and where the cleat engages. I would also suggest that you develop a routine, e.g., by unclipping as you come to a stop with the same foot each time. I would agree, however, that if you still feel shaky with them by the time of the ride, switch back to your platform pedals.
I also recommend that you start riding on the road in advance of the ride. I would start with doing some research on the rules of the road that are specific to your state/city. From there, see if you any of your friends would be willing to go for a road ride with you. Since it sounds like some of your friends are experiened cyclists, they could give you some pointers for dealing with traffic and could also introduce you the rules of etiquette for riding with a group, e.g., how and when to use hand and voice signals. You might also think about riding with a bigger group, e.g., at a local club ride, before doing your event ride.
When I first started riding, I remember how much concentration it took to just ride. When did it get so complicated, I remember asking myself. It seemed so intuitive when I was a kid. It was hard to devote the attention necessary to other riders and traffic. After a relatively short time, however, I found myself not having to work so hard at it and now, much of it seems like second nature. Just be patient with yourself, and you'll get there too in no time.
Have fun.
Kate
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher