Jolt, I understand your concerns, really I do. I rode my road bike virtually by myself for several months before going to a club ride because I didn't know what to expect. Admittedly, those rides involve lots of people riding at any pace from 12 to 25 mph so I had to know something about bike handling to ride safely. However, in retrospect, I totally overthought the whole thing and was much more anxious than I needed to be. I'd suggest the same thing to you. Go, ride, be careful, have fun. This doesn't sound like a hard core group of cyclists and they're probably used to having new riders on all sorts of bikes at all sorts of skills levels.
As far as how close people ride together in a group. It depends on the group and the type of ride. On a 12-mile , 14 mph ride, however, I would assume that you won't be riding inches from one another's wheels. You'll probably be in a loose bunch if the street conditions allow it. On busier roads, you'll likely ride single file. Just keep the distance that you feel you need to ride safely and comfortably. Just don't get too close to anyone's wheel. If you get to a hill, just ride your pace and keep to the right side of the road so that faster riders can pass you on your left.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher