Head-mounted displays (HMDs) increase immersion into virtual worlds. The problem is that this limits headset users' awareness of bystanders: headset users cannot attend to bystanders' presence and activities. We call this the HMD boundary. We explore how to make the HMD boundary permeable by comparing different ways of providing informal awareness cues to the headset user about bystanders. We adapted and implemented three visualization techniques (Avatar View, Radar and Presence++) that share bystanders' location and orientation with headset users. We conducted a hybrid user and simulation study with three different types of VR content (high, medium, low interactivity) with twenty participants to compare how these visualization techniques allow people to maintain an awareness of bystanders, and how they affect immersion (compared to a baseline condition). Our study reveals that a see-through avatar representation of bystanders was effective, but led to slightly reduced immersion in the VR content. Based on our findings, we discuss how future awareness visualization techniques can be designed to mitigate the reduction of immersion for the headset user.