Basically they give you two of them that are connected and let you send a vibration (for example) to the other owner.
What kind of communication do they use? Do they have a sort of Sim card? Would it be possible to make a similar product with Arduino (I was thinking about Arduino Gemma)???
HEY comes as a set: two bracelets that are linked to each other via Bluetooth and an app on your smartphone.
I suspect it should be possible to implement something like that using Arduinos and bluetooth modules. Are you sufficiently experienced to write the necessary smartphone program?