Ask HN: Geolocation(HTML5/PHP): Tutorial on plotting numerous users on map?

Can someone can refer me to a tutorial or such that provides information regarding how to capture a users location and plot other users around them? I'm hoping to build an app that contains a functionality for you to open the app on the phone and see other users within a set radius that are also running the app. I'm using HTML5/PHP, and I'm newbie to this, so I'm assuming I'd need to use Google Geolocation APIs or such to pass/capture multiple user locations and plot them? All help is greatly appreciated. Thanks for your time and assistance.