The research presented in this dissertation focuses on multi-user VR, where multiple immersed users navigate the virtual world by physically walking in a large tracking area. In such a setup, different combinations of user colocation within the physical and the virtual space are possible. We consider a setup to be multi-user if at least one of these two spaces is shared. The dissertation starts with the classification of combinations of physical and virtual colocation. Four such combinations are defined: colocated shared VR, colocated non-shared VR, distributed shared VR and shared VR with mixed colocation. The characteristics of each of these four setups are discussed and the resulting problems and research questions outlined. The dissertation continues with the description of ImmersiveDeck - a large-scale multi-user VR platform that enables navigation by walking and natural interaction. Then, four experiments on multiuser walkable VR developed with the use of ImmersiveDeck are described. The first two experiments are set in colocated non-shared VR where walking users share a tracking space while being immersed into separate virtual worlds. We investigate users mutual awareness in this setup and explore methods of preventing mutual collisions between walking users. The following two experiments study shared VR scenarios in situations of varied physical colocation. We investigate the effects that different modes of physical colocation have on locomotion, collision avoidance and proxemics patterns exhibited by walking users. The sense of copresence and social presence within the virtual world reported by users is investigated as well.