Objectives: Better engagement of diverse groups of interest-holders in the development of health guidelines has been proposed to improve their usefulness, implementability, and acceptability. Guidelines shape clinical or public health practice decision-making. Trustworthy guidelines are systematically developed documents that include actionable statements based on evidence and a formal, structured and transparent decision process. This paper describes the Guidelines International Network (GIN)-McMaster Guideline Development Checklist (GDC) Extension for Engagement to assist developers with engaging multiple interest-holders throughout all topics of guideline development. Study Design and Setting: To produce this checklist extension, we conducted a 3-phase mixed methods study. First, we identified 10 groups of interest-holders to be engaged in health guideline development: patients, the public, providers, program managers, principal investigators, payers/purchasers of health services, payers/funders of health research, policymakers, peer-reviewed journal editors, and product makers identified in previous work and recruited coleads to represent these groups (n = 26 total). Results: We conducted a series of reviews to identify existing methods and barriers/facilitators for engagement, approaches to managing conflicts of interest, and describe the impacts of engagement on the guideline development process. The results of these reviews informed the development of an online survey for which we received 195 responses. We clarified these results through 43 key informant interviews with interest-holders. The final GDC extension checklist was determined based on consensus methods with our coleads. Conclusion: This paper presents the GIN-McMaster GDC Extension for Engagement. This checklist provides interest-holder–informed recommendations for providing advice/feedback or participating in decision-making in guideline development. Plain Language Summary: Health-care guidelines are formal recommendations for diagnosing and managing medical conditions or public health concerns. The steps involved in developing guidelines include from planning the process, defining the focus of the guideline and the questions it should answer, and summarizing the evidence. The Guidelines International Network-McMaster Guideline Development checklist includes 146 steps of guideline development grouped into 18 topics. Our work extends this checklist to provide guidance on how to engage with people from different groups of ‘interest-holders’ throughout guideline development. These interest-holders include (1) patients, (2) members of the public, (3) health-care providers, (4) people who manage and operate health programs, (5) researchers, (6) people who pay for or purchase health services, (7) policymakers, (8) people who pay for health research, (9) people who publish research and guidelines, and (10) people who develop and sell health-care devices or products. We engaged with people from each of the groups to develop this guidance. Twenty-six people coled our interest-holder groups. We conducted a series or systematic reviews on (a) how to engage people in guidelines, (b) identifying the barriers and facilitators to engagement, (c) assessing the impacts of engagement, and (d) describing how to manage conflicts of interest related to engagement. These reviews were used to create a first draft of guidance. Then we conducted an online survey and gathered the opinions from 195 people from each of our interest-holder groups. We interviewed 43 interest-holders for additional information to revise the guidance and then finalized the guidance with our interest-holder coleads. This checklist provides guidance for engaging interest-holder groups throughout guideline development.