[shshhdhs]

I absolutely love how simple and clever this is, from a security/privacy perspective. The display of the number of folks in your "group" is pure fun. =)

I kind of wish this could be an app. Anyway to replicate the effect on iOS or Android? Or do they not expose the necessary APIs?

[javl]

One of the things you'll need is a network card with the ability to go into monitor mode, which allows it to see all traffic instead of only traffic meant for your device. I don't think there are a lot of mobile phones that have such a card (the Nokia N900 did).