User participation is often reported as a success factor for public e-service development. However, in practice, project managers face challenges to involve users due to lack of methodology, time, or context awareness. In order to tackle this lack of context awareness, we describe a conceptual model summarizing the impact of four influencing factors on user participation decisions. From this model, we then derive a decision support guide that helps project managers from public organizations understand when and how to include users in the development process. Following the design science research approach, the conceptual model is built in an iterative manner based on several literature sources and validated through in-depth interviews and group discussions with project managers and researchers. This paper contributes to research as it helps to frame the conditions impacting participation and opens the discussion for the addition of other factors. Then, it also contributes to practice as project managers developing public e-services will be able to reach a better situated participation.