(Hint: potential users count.)
Actually this could look really cool!
You could use bluetooth, but then you have to switch it on.
You could use the camera to pick up the phones behind you somehow, but that would not always work.
Thar be dragons down that path... Users don't know what they want:
Hopefully, I will hear from users soon. If not, I guess I'll have to make it a rave toy as suggested by the people on this forum.
EDIT: Maybe it's not up that high. The rankings in the App Store all seem to have shifted all of a sudden, and the top 20 apps all look wrong.