BackgroundThe Peer Engagement and Evaluation Project (PEEP) aimed to engage, inspire, and learn from peer leaders who represented voices of people who use or have used illicit substances, through active membership on the ‘Peeps’ research team. Given the lack of critical reflection in the literature about the process of engaging people who have used illicit substances in participatory and community-based research processes, we provide a detailed description of how one project, PEEP, engaged peers in a province-wide research project.MethodsBy applying the Peer Engagement Process Evaluation Framework, we critically analyze the intentions, strategies employed, and outcomes of the process utilized in the PEEP project and discuss the implications for capacity building and empowerment among the peer researchers. This process included: the formation of the PEEP team; capacity building; peer-facilitated data collection; collaborative data analysis; and, strengths-based approach to outputs.ResultsSeveral lessons were learned from applying the Peer Engagement Process Evaluation Framework to the PEEP process. These lessons fall into themes of: recruiting and hiring; fair compensation; role and project expectations; communication; connection and collaboration; mentorship; and peer-facilitated research.ConclusionThis project offers a unique approach to engaging people who use illicit substances and demonstrates how participation is an important endeavor that improves the relevance, capacity, and quality of research. Lessons learned in this project can be applied to future community-based research with people who use illicit substances or other marginalized groups and/or participatory settings.