The hardest thing to do is ride at the back of a large group. The 'elastic' gets streched with every gust of wind, slight climb / descent, or intentional change in pace.
As another poster mentioned, being fit enough to ride with the experienced riders is very helpful, you'll learn alot from these guys.
Prehaps your local area has a 'recovery ride' day? Here, on Saturday we go out and smash ourselves, Sunday is an easier ride, usually riding 30 - 34kph while on the flats, for about 80km. We wait at the top of the big hills. If you aviod Saturday you might have enough in your legs to be able to ride near the front of the Sunday bunch, and take your turn on the front. The easier pace and larger gaps between wheels makes it easy to learn the techniques you need, and the social setting means you might actually enjoy yourself
If there is something you're not sure about, don't be scared to ask one of the obiously experienced riders, better to ask and look like a fool than to take down the whole bunch and BE a fool. There is usually at least one 'vocal' rider in the bunch who might be quick to point out your mistakes. Generally these guys are doing it for the good of the bunch, not to make you feel outcasted so dont take it too personally. If you end up riding next to the guy who told you off, ask him what you did wrong and what you should have done, its part of the learning process.