One of the eventual goals of VR Juggler is to provide full support for multiple users. The first step is the Juggler User chunk, which currently associates a positional input proxy with a user's head. You'll need to create at least one of these, to use as a target of your Display chunks.

This singleton Chunk defines a few properties needed for using Iris Performer-based VR Juggler applications. If you aren't using Performer, you don't need to worry about this one.