Currently, "walkable" virtual reality (VR) is achieved by dedicating a room-sized space for VR activities, which is not shared with non-HMD users engaged in their own activities. To achieve the goal of allowing shared use of space for all users while overcoming the obvious difficulty of integrating use with those immersed in a VR experience, we present ShareSpace, a system that allows external users to communicate their needs for physical space to those wearing an HMD and immersed in their VR experience. ShareSpace works by allowing external users to place "shields" in the virtual environment by using a set of physical shield tools. A pad visualizer helps this process by allowing external users to examine the arrangement of virtual shields. We also discuss interaction techniques that minimize the interference between the respective activities of the HMD wearers and the other users of the same physical space. To evaluate our design, a user study was conducted to collect user feedback from participants in four trial scenarios. The results indicate that our ShareSpace system allows users to perform their respective activities with improved engagement and safety. In addition, this study shows that while the HMD users did perceive a considerable degree of interference due to the internal visual indications from the ShareSpace system, they were still more engaged in their VR experience than when interrupted by direct external physical interference initiated by external users.