it never seems to be a good idea for a beginner to join up with a group on a weeknight ride in the middle/end of the racing season. everyone is all hopped up, excited and in peak physical condition, adrenaline is going, people want to go fast. winter and fall are always the better times to join up with a team or club as a beginner, so don't feel too bad about getting dropped at this point..
example: there is an open ride on the weekends here that is a "fast" training ride that is moderate in the winter time, but in the summer and spring, pacelines average above 30 miles an hour on the flats. thats fast.. but then again, everyone has been cruising all winter, and when racing season starts, EVERYTHING changes.. likely in a few months, those rides you went on will quiet down.