It doesn't really matter how fast "fast" is. The OP just wanted to get faster so that she can hang with group A, because group B doesn't really challenge her. No matter where your starting point is, I will still say that intervals and plain old time on the bike will help your fitness and speed improve, but riding with faster people is the quickest and most effective way to really challenge yourself so that you see some speed improvements sooner. "Sooner" may still mean months, which is why you don't necessarily want to ride with the superfast crowd all the time--that gets demoralizing. But it is good for you if you want to get faster. You just need to pick a group that you can *almost* hang with--whether everyone would consider them "fast" is irrelevant. They are just that one step faster than you at that time. Maybe it's the "A" level group, maybe not.