In fact, Page speculates, grouping only smart people together doesn't work that well because the smart people (whatever that means) tend to resemble each other in what they can do. If you think about intelligence as a kind of toolbox of skills, the list of skills thy are "best" is relatively small, so that people who have them tend to be alike. This is normally a good thing, but it means that as a whole the group knows less than it otherwise might. Adding in a few people who know less, but have different skills, might actually improves the group's performance.