Yes, riding a straight line is crucial in a group ride and an invaluable skill to have overall. Many newer riders tend to weave around a lot so if you can learn to ride a straight, predictable, even line you'll be ahead of the game and much less likely to have an unfortunate encounter. You can practice riding straight by following the painted line on the side of the road (in the right road conditions) or even a line painted in a big parking lot. It's only a couple of inches wide, but try to keep your wheel entirely on it for as long as possible.

The other thing I would say, if you haven't had a lot of experience riding on the road, is to be aware of potential accident scenarios involving cars. They're much easier to avoid if you've thought about them beforehand. An ounce of prevention is definitely worth a pound of cure in this area. A couple of weeks ago I put up this link to a site that gives some very good basic information about this topic.

http://bicyclesafe.com/

Finding bike rides in your area is sometimes as easy as doing a Google search for your city name plus the words "bicycle clubs" or "bicycle rides". I would say the overwhelming majority of bicycle clubs these days have websites and many list their rides for all to see. (My local club makes you get a free user name before you can see the ride calendar.) They usually also include contact information for the ride leaders so if you need to contact them about speed or difficulty level that's easy enough to do. And keep in mind, in general anyone willing to volunteer as a ride leader is usually a pretty nice person to begin with, because it is sometimes a thankless task. They don't always get the credit they deserve.