Another thing: Make sure you know where the ride "is on." A lot of the rides around here have neutral roll-outs, so the first few miles from the meetup are at mellow, conversational pace, and then at some point (it may be well-defined, it may not), it becomes a race. Here, the Tuesday Night South River Ride is neutral for like 8 miles as it winds through neighborhoods, but once it crosses the bridge into Yolo County, it's go time. First time I went, no one told me that, and I got dropped instantly. So don't be afraid to ask.
Also make sure to ask where the sprint and regroup points are, if any.