Citizens are increasingly using Open Government Data (OGD) and engaging with OGD by designing and developing applications. They often do so by collaborating in groups, for example through self-organized groups or government-induced open data engagement initiatives, such as hackathons. The successful use and engagement of OGD by groups of citizens can greatly contribute to the uptake and adoption of OGD in general. However, little is known regarding how groups of citizens develop in OGD engagement. This study aims at exploring and understanding the development stages of citizen groups in OGD engagement. To attain this objective, we conducted a comparative case study of group development stages in two different types of OGD engagement. Our cases show that leadership and diversity of capabilities significantly contribute to the success of citizen groups in OGD engagement. These findings suggest that connecting citizens having a diversity of expertise prior to the OGD engagement event helps to improve its effectiveness. This research is among the first to apply group development stages model in open data engagement studies and thus opening up new research opportunities concerning group developments in the open data literature.