Studies on open innovation have increasingly emphasized the role of individual users as collaborators in the innovation processes. Keeping users enthusiastically engaged throughout the information systems development (ISD) process for open innovation approaches is of crucial importance, specifically when the participation is voluntary. Although a few studies exist that have presented process models for user engagement, none of them are specifically based on the voluntary contribution of the users in an open environment, nor included the detailed attributes of the phenomenon of user drop-outs in their presented models. By combining the results of a comprehensive literature review and a qualitative case study of user engagement, this study aims to develop a user engagement process model that includes the variety of reasons for user drop-out, and then to present some practical guidelines that would facilitate prolonged user engagement in the ISD process in an open innovation environment.