create a contact in your phone with the message you want ("Look behind you," "the call is coming from inside the house," "Those shoes with that shirt? reallY?!", "look up NOW!", "Don't drink the punch." etc... )

Then choose to send that contact by Bluetooth. Your phone will then search for unwilling participants' cell phones that are discoverable. Pick one.

One note: if you named your phone in Bluetooth, make sure it's not your name or you might be given away!