In the two experienced group rides that I've been a part of, I used my steel beater. I was barely able to hang on to the pack, but people seemed impressed, and told me I "ride good". I'm not sure if it's because it was my first time, or because it was my steel bike. But one guy pointed out that if I rode clipless, with a better bike, I would be kicking their butt. I can see where they're coming from, considering being able to accelerate really fast plays a big role. but for now, I'll let the engine do all the talking.
Tomorrow, I'm bringing my good road bike (well not really because it's like 23 lbs vs my 26 lbs beater). By next week, my new kestrel carbon bike arrives, and after that, a good pair of road shoes with spd-sl pedals. I'm sure it'll matter one way or another. The question is how much it matters. My take on it is that a good fit is most important.