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?
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).
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?