Do you mean your first ORGANIZED large ride, or actually the first time you have ridden with a group of people?

If it is the second, I would recommend you find a riding club in your area. Riding in a group is great, but there are things about it that are just different from riding by yourself. You will get to practice some of the things people here have mentioned, calling out road hazzards, keeping predictable, etc.

I did my first organized ride with a friend who could have ridden much father and faster without me. The second one I did I was 'alone'. There are great things about both ways, for me. Having Tod there was a comfort for me, if anything went wrong I had someone I knew and trusted to help, but I also felt like I was always holding him back. Going by myself gave me the chance to just ride my ride. I didn't have to worry about keeping up with a faster rider, and didn't have to feel guilty about holding someone else back. I moved in and out of groups the entire day, chatting with strangers like they were family. By lunch time I had plenty of options of people to sit with, as I had talked to dozens of people on the way. There did come a time when I was alone for a couple of miles...totally alone, could not see riders in front or behind (the support crews kept coming by to check on me though). That actually was kind of nice too. I knew that no one was waiting on me, worrying about me, and i could just concentrate on pushing on.

Good luck on your ride!