This is best done during the safe room visits (end of the days in multiplayer) while everyone is standing around or gathering supplies within the small area. Simply push
for camera and
for Selfie mode. You just need to get everyone in the shot and it'll unlock, I'll also mention that it highlights all captured members with a square in the picture. It's also easier if you discuss needing to get it with everyone, but even without that you should be able to get it in the safe room portion of multiplayer.