The topic of safety on group rides has come up on several mailing lists this week.  Here are some great tips (PDF) for folks riding in groups.  Most important - don't fall into the herd mentality and forget to obey all traffic laws.  This includes stop signs, traffic lights, signalling, etc.  Remember that when you ride in a group you become an even bigger ambassador for cycling and automobile drivers will notice your behavior even more than when you ride solo.