In this paper, we set out to explore how to design for spontaneous group formation, as part of shared encounters in a public installation. Spontaneous group formation describes a phenomenon where pedestrians form groups with other pedestrians, to whom they are previously unacquainted, for the purpose of interacting with an installation. This was accomplished by developing a 5 by 5 meter version of the game based on the Simon game, with flashing lights and oversized buttons, designed to encourage spontaneous group formation by giving an advantage to larger groups, up to a maximum of four. Over three days of testing, the prototype was found to generate 11 spontaneous group formations out of 161 total use cases, showing evidence that effective lures are one of the key factors behind the phenomenon.