another thing to consider when moving up to faster, more experienced rider groups, is your own level of awareness, understnading and confidence.
Loose groups, where you're riding at a very conversational pace, are a whole different thing from groups which maintain a faster pace, tighter formation and surge more often.
Be quite sure of your group riding skills, as relates to knowing how to hold a wheel, hold a steady line, how to read riders ahead of you and any number of other skills important to both your and the others safety. If it's an experienced group, they'll use hand signals and not much/any verbal - a good thing to acquire quickly.
That said, there's nothing like giving it a go. But be prepared to tail off if you start feelin sketchy. It's better to do partial rides until you get comfortable with needed new skills. Watch the guys who are obviously well skilled and experienced. Work on steady and preditcable more than fast. But don;t be afraid to push yourself when the goin gets tough.
You're in a group, yes, but remember only you are responsible for your ride.