This research deals with an analysis of forms of participation in a participatory design (PD) process of a software that assesses the sustainability of agricultural cropping systems. We explore the actual forms of participation of designers and users by adapting an Actual Role Analysis in Design approach (Barcellini et al., 2013) to capture the levels of abstraction (conceptual, functional and operational) of participants' discussions. We show that: (1) the process does not only concern the design of the artifact itself, but also the design of the concept of sustainability; (2) all participants (users & designers) have a role in co-designing the concept (in our case, sustainability); (3) some roles and profiles are key to this co-design. We discuss our contributions to both the research and the practices of participatory design. These contributions deal with the production of a method and related knowledge about actual activities in participatory design situations. They may support the development of relevant training programs regarding participatory situations, or be reflexive activities that can help those who are involved in designing and leading in participatory situations, to make improvements.