What happened!?
Most racer-oriented group rides seem to be a free-for-all, and sometimes it takes years to figure out what's going on. Routes, length of the warm up, sprint locations, drop policy, pacing, and more are largely a mystery. What's just part of the fun for one rider is a faux-paus to another. Even riders who come often don't know the plan.
So what is the purpose of a group ride?
That's the main question that rarely gets answered—out loud anyway. Everyone shows up with their idea of what they wish or expect the ride to be: hard workout, speed work, paceline practice, sprint training, race simulation, testing your (or your buddy's) fitness, cojones measurement, etc.
Bicycle touring/recreation clubs often pre-define multiple groups by pace and drop policy. Not a bad plan, but racing-oriented group rides aren't often that organized. Whether you are new to the group ride or have been coming for years and are still looking for answers to the mystery of your group ride, here are a few suggestions.
Be prepared.
That's the Boy Scout motto. Know the start time and location of the ride, and the length of the ride, if possible. Bring plenty of food and fluids, plus your own tools for roadside repairs. If you have a good, compact map of the area (or a phone with GPS/maps), bring it along if you are worried about getting dropped and being lost.
Arrive early.
Get to the meeting point before the scheduled ride time so you have time to get your bike ready, meet a few people, and inquire about the plan for the ride. As a new rider, you don't want to be rushing in last minute and chasing everyone out of the parking lot without having met anyone or asking any questions about the route or type of ride to expect.
Ask questions.
Ask about the planned route, warm up, structure of ride (organized, free-for-all, drop policy, etc.). Don't be discouraged if you don't get the answer right away—you may not be the only person who doesn't know the plan. Ask multiple people, if necessary.
Find the leader.
Usually there is one person who is the appointed or de facto ride leader. This person does not always have the answers either, but by asking this person about the pace, structure, warm up, sprints, and drop policy, you just made it more likely that the ride will have more of a plan.
Make a friend or two.
Every group has people who are naturally friendly and more than happy to help. Talk to a couple of people and find a person or two you are comfortable asking questions during the ride, like how far to the next sprint. If you are worried about getting dropped along an unknown route, having a friend to remember you is good, especially in the event you have a mishap.
People do group rides for a variety of reasons: comroderie, fitness, learning skills, and much more. But just like team racing requires communication, so do group rides. So talk about it. Ask questions. That goes for riders new to the group and riders who have been showing up for months or years but don't know the plan. If you happen to be the leader of the group ride, keep all of these questions in mind and even consider giving a quick pre-ride plan to everyone regarding the route and goals for the day.
A last note for all of the "usuals" on group rides, please say "Hi" to the new guy. Find out his name. Let him know what normally happens on the rides and ask if he knows the area (especially if no one waits for dropped riders). So many group rides are like an exclusive club where anyone can show up, but only the special people get in. Don't force new riders to finish with the front group before they even get a "Hi, how are you? What's your name?"