UPDATE: “Project Athena” has been renamed to “Vircadia”.
The Radar app is now released for Project Athena. See: Project Athena > Radar for app details.
The Radar app was originally released on the High Fidelity Marketplace but the Marketplace is no longer available now that High Fidelity has suspended their work on their virtual world platform. Project Athena has taken the torch and is continuing development on the open-source, VR capable virtual world platform that High Fidelity started. The Radar app has been re-released for Project Athena.
You can run or download the Radar app from the links provided at Project Athena > Radar.