You need to find someone in a crowd, why not use your smartphone as a beacon.
Just agree upon a color in advance and hold your phone up in the crowd with
one of these flashing colors so your friends can locate you.
Hold down on the flashing to make the phone go all white to further customize your signaling.
This is a web based clone of the app LookFor. If you need to signal in remote locations where there is no data connection download their app
This is in beta, feel free to send feedback