For better results, diverge first: allow each member to make his or her guess before being influenced by the group and, most importantly, the boss. We want an uncorrupted look at what each person believes to ensure the greatest possible diversity of thought. A simple way to do this is to ask each person to write down an estimate before any discussion occurs. Now converge: review the estimates as a group without identifying who made each one, and then narrow the possibilities down collectively.
