Everyone has already talked about the pace and getting over your head, so I will mention something else. Another thing to keep in mind when group riding is the guy behind you is counting on you to pass along important info from the lead. There are important hand signals that only can be learned through practice and experience. Noticing when there is need to slow down or an obstruction in the road. All this info needs to be passed along to the rider behind you. This will come in time when you ride in a small group with experienced riders, that is how I learned.