This can be done at any point after the beginning of the game. Simply open the camera app in your phone and put it on selfie mode. Now walk down a busy sidewalk, pointing the camera at various NPCs. Randomly one will react by posing with you.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKcpUn58waV...