Appealing participants' engagement drives collaborative systems to enhance it through system's use or through system's design. However, engaging participants in collaborative systems to create digital resources is not trivial to achieve as the majority of contributions are provided by a very small percentage of engaged participants. In the literature, different approaches, such as human-in-the-loop and co-design, investigate engagement in these lines. This thesis aims to study how reflexivity can help designers to investigate participants' engagement in co-design of collaborative systems. Based on a qualitative approach, the thesis is positioned in the field of Human Computer Interaction and grounded on two studies. The retrospective analysis of the two studies was guided through a framework composed of three phases. In the first phase, supported by the literature review, several qualitative methods were investigated to identify the communities to be involved in the research; in the second phase, different co-design sessions were conducted with participants; and in the third phase, participants evaluated the solutions co-designed. The two studies followed different but intertwined approaches. Study 1 followed a user-centric approach and supported the identification and consolidation of a set of factors that hindered or facilitated engagement. The factors were articulated as barriers, drivers, and workarounds, and were validated in Study 2, which followed a participative approach. These factors constitute the first contribution of this thesis. Moreover, the literature review and the empirical data supported the identification of three dimensions to facilitate the adoption of a reflexive approach in co-design. These dimensions correspond to the second contribution of this thesis. Finally, the set of barriers, drivers, and workarounds was merged with the dimensions to propose a framework to investigate engagement in co-design of collaborative systems, constituting the third contribution of this thesis.