Android Question Send Link

It is impossible to send anything to someone without knowing some addressable ID for them like:

1) Phone number
2) Email
3) Username for an app that is *already* installed on their phone

Otherwise, the other people in the room will first need to do some action (like install an app, scan a QR code, or go to a URL), before you can communicate with them.

Do they connect to a common WLAN?
Another point to investigate: if you set up an hotspot/AP, do their OSes knock on its door in order to have an updated list on wi-fi allowed? If yes, what kind of info is trasmitted to your device? Here I don't mean they have to conenct to that AP, just scan for any available.