As the title suggests, does anyone know how to add multiplayer into an HTML5 Android game?

It depends on your gameplay, but basically, it’s using a server to centralize and process the data. There are several languagues/technologies available, but the HTML5 have to send request to the server, whatever it’s using(php/js/etc).

The simpliest (but i’m not telling it’s simple) is to use php and mysql database, but not really adapted in a lot of case : it couldn’t be easily used to sync 2 player in realtime game.

Check my signature for watch a demo about multiplayer with nodeJS