After watching the opening ceremony at the Beijing Olympics, I'm reminded of how a different beauty emerges from large masses of people as from groups and individuals. Over two thousand drummers synchronize their actions to create an awesome spectacle that's simply not possible in any other way.
Games are increasingly able to include vast numbers of objects and characters -- coordinately directly or indirectly, or acting separately. Keep in mind that sheer numbers can change aesthetic and narrative possibilities. Through masses, you can create experiences for players that are essentially different from encounters with individuals and groups.