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This article analyses the participation mechanisms of people with experiential knowledge of different realities, including mental health problems, homelessness, addiction and poverty, in the Chez-soi Montréal project (2009-2013). This analysis allows us to characterize their role within the project, in and outside the committees in which they were taking part. We focus on the idea of professional monopoly to explain the relatively superficial involvement of peers in this project, as well as on factors that limited a greater sharing of decision-making, even though this sharing was desired by several actors within the project. Through this analysis, we illustrate and contextualize the issues that stand out in the literature on the participation of users and former users of health and social services in health and social services research and intervention projects.