We present preliminary results of work on a low-cost multi-user immersive Virtual Reality system that enables collaborative experiences in large virtual environments. In the proposed setup at least three users can walk and interact freely and untethered in a 200 m(2) area. The required equipment is worn on the body and rendering is performed locally on each user to minimize latency. Inside-out optical head tracking is coupled with a low-cost motion capture suit to track the full body and the head. Movements of users, 3D interactions and the positions of selected real world objects are distributed over a wireless network in a server-client architecture. As a result, users see the effect of their interactions with objects and other users in real time. We describe the architecture of our implemented proof-of-concept system.