We always say, never paddle alone and for good reason.  If you should pin your boat or even end up swimming and injure yourself, you will often need the help of others.  When paddling in a group, it is important to be a team player.  Keep an eye out for a couple of paddlers in front and behind.  Prior to putting on, it isn't a bad idea to discuss potential group gear like a breakdown paddle and a decent first aid kit.  Are there any weak paddlers that need to be shepherded?  Perhaps you can help out in some way.  The trip leader should try to keep the group together.  This requires periodic regrouping.  Weaker paddlers should be moved to near the front of the pack whenever possible.  When you come to particularly difficult rapids, resist the urge of talking someone into running a rapid they don't feel comfortable with.  Group peer pressure is a real big no - no.  Every paddler needs to decide what is best for them on that particular day.  Here is a nice article on judgment: Learning Judgment.