This article reports a study on how international English language learner (ELL) students participated in a collaborative learning community comprised of peers and peer tutors in a writing center in a U.S. university. Drawing on the theory of community of practice (CoP), qualitative data were collected from weekly observations of three groups and in-depth interviews with four ELL students and six tutors. Findings suggested that these three groups functioned differently as CoPs with varied levels of participation and social interaction among the students and between students and tutors. A mismatch between the goals and roles of the students and those of the tutors, and group dynamics among students and between students and tutors were discussed. Implications for practices and research to better support international ELL students were discussed.