Faster more experienced riders to the front. Newbs to the rear. Passing should only take place to correct the riding order. And really you shouldn't have to pass, if you are holding up the rider behind you wave him by. Stop to count bikes before turning onto a new road or at a major junction. If the group contains really new riders, or someone who has never ridden a particular road before, then put an experienced rider in the sweeper position. Rinse. Repeat. A new rider is more likely to push to if he knows he is holding up a veteran, rather than falling behind if it's explained that we stop at count bikes at every turn. Seriously, if you are going to ride over your head to try to keep up with one of the fast guys, you should think really hard about whether or not you belong on a group ride. Not only should you be comfortable withthe group, but the group should be comfortable with YOU. I have left group rides to ride solo, when I can feel the inevitable crash coming, despite a rider being asked to ride within his limits.