A real-time collaborative assembly modeling process involves the teamwork of multiple designers. Without adequate coordination, this multi-user based modeling process could be more time consuming, or even divergent, than the conventional single-user-based process. This paper thus presents a supervisory control approach to achieving online operational coordination of the multi-user based assembly modeling process. In this approach, we treat the real-time collaborative modeling process as a discrete-event system (DES) and then obtain an effective coordinator for the process control via the supervisory control theory (SCT). Our work extends the framework of SCT to this new application so that the assembly modeling operations and its desired operational behaviors can be modeled and controlled by a set of automata. With them, we further propose a modular supervision approach to find a group of modular supervisors. These supervisors compose the online coordinator to enforce the control specifications and to yield a nonblocking controlled process. The results show much promise for SCT in the new application domain of collaborative CAD (CCAD).