it tends to be difficult to keep everyones attention for very long.
Having one or more people appear in a cirkut photo by running around is actually a good way to keep people's attention. I shot my daughter's fourth grade class on a field trip, and had the teacher and a lot of the class run around.The teacher was in the shot 4 times by running behind the kids. they had a great time, and it kept their attention for the shot.
Good planning and presetup is also necessary. you don't want to bring the people over till you are mostly ready for them. Also , when you are shooting large groups you have to expect some people looking around. most do pay attention. it's fun to do. I tend to be a bit shy, but after the first hundred people, you have to let go and hope everyone is doing what they are supposed to.
Jamie Young