There has been growing interest among learning scientists in the design and study of out-of-school time (OST) learning environments to support equitable development of science, technology, engineering, and math (STEM) interests among youth from groups that are underrepresented in STEM fields. Most of these design studies assumed the youth came to the learning environments without well-developed STEM interests. I challenged this assumption by enacting a social design participatory study to engage youth (aged 11 to 14), from groups that are underrepresented in STEM fields, as partners in designing an OST networked club to support the youth in growing their own STEM interests. Based on longitudinal ethnographic data, I report a three-year iterative design of this networked club. I characterize the heterogeneity of STEM interests that emerged and grew across the networked club. Building on ecological theories of interest development, and leveraging the cultural assets of the nondominant community, I argue that heterogeneity of interests, resources, and practices served as a design tool and a catalyst for the development of STEM interest in the OST networked club.