Group rides are a lot of fun so I'm sure you'll have fun. Since it's your first time riding in one the best thing to do is control your bike properly and not do anything that will endanger the other riders; focusing on speed is something you can do once you're comfortable riding around other cyclists and can pace yourself properly. There is no shame hanging off the back and paying attention to what other riders are doing, and chat up some of the more experienced riders who will be a great resource. Enjoy!