An emerging body of literature has noted the salience of subgroups in sport, emphasizing important implications for athletes and teams (Martin et al., 2020). With the advancement of a conceptual framework for subgroups (McGuire et al., 2021), a warranted next step is to advance a psychometrically sound questionnaire. Measuring subgroups is nevertheless complex, given the need for an innovative tool that describes not only the distribution of teammates into subgroups but also perceptions of how subgroups relate to member behaviour. Thus, a collaborative, iterative, and critical process that differs from how many self-report tools are developed is needed for measurement development. Purpose: To initiate the development of a subgroup measure in sport. Methods: A three-phase process was adopted that involved literature/questionnaire reviews and research team collaborative meetings (Phase 1), topic expert document review and semi-structured interviews (Phase 2; N = 5), and athlete think-aloud interviews (Phase 3; N = 7). Results: Phase 1 resulted in a proposed subgroup questionnaire, with Phases 2 and 3 leveraging the knowledge of experts and experiences of athletes beyond the approval of items for representativeness and readability. Specifically, changes to the questionnaire structure (e.g., context descriptions) and participant orientation were made. In addition, considerations for including more in-depth discussions with experts and athletes will be put forward (e.g., discussing expert intention and conducting virtual think-aloud interviews). Implications: This study contributes to the advancement of the subgroup literature and provides an example of how expert input can be further incorporated into questionnaire development protocols.