Infrastructuring Pop-Up Cities with "Social Layer": Designing Serendipitous Co-Livings for Temporary Intentional Communities

After the pandemic, a new form of "pop-up city" has emerged -- co-living gatherings of 100-200 people for 4-8 weeks that differ from conferences and hack houses. These temporary intentional communities leverages existing urban infrastructure, blending daily life (housing, meals, care) with self-organized activities like learning, creating, and socializing. They coordinate bottom-up programming through an "unconference" system for identity, calendaring, RSVP, and social discovery that fosters spontaneous, serendipitous, enduring ties. This paper examines the design of "Social Layer," an unconferencing system for pop-up cities. We studied its real-world deployment in ShanHaiWoo (Jilin, China, 2023), muChiangmai (Chiangmai, Thailand, 2023), Edge Esmeralda, Edge Esmeralda (Healdsburg, CA, USA, 2024), Aleph (Buenos Aires, Argentina, 2024), and Gathering of Tribe (Lisbon, Portugal, 2024). Our findings distill: (1) the strong concept "scaffolded spontaneity" -- infrastructural affordances that balance structure with openness, amplifying participant agency while maintaining privacy and lightweight governance; (2) design implications for design researchers working on pop-up cities.