Learning communities (LCs) can provide authentic, social learning experiences but require an extensive amount of time and effort to orchestrate, often more than instructors can provide in typical university courses. Extracurricular, undergraduate, student-led learning communities (SLLCs) overcome this cost through volunteer peer-instructors. Unfortunately, LCs theory is based exclusively on teacher-led LCs. Here we ask "what instructional processes emerge in SLLCs?" We conducted a qualitative case study of SLLC student leaders' attempts to teach a project management practice (StandUp) to student innovation teams. We found that instruction in SLLCs takes the form of a bi-directional diffusion process, in which peer-instructors influence students' decisions about what practices to participate in, and students influence peer-instructors' decisions about advocating for practices. Three major findings support the bi-directional diffusion model. First, students' participation in StandUp hinged on whether they saw the practice as valuable with respect to their social, learning, and/or performance goals. Second, peer-instructors struggled to persuade and scaffold students to participate in StandUp. Third, students influenced peer-instructors to stop advocating for StandUp. The bi-directional diffusion model highlights the practical importance of persuading students to participate in the community's practices. The model suggests that we might support peer-instruction by promoting peer-instructors' content knowledge about practices, their persuasion skills, and their motivation to advocate for practices.