I'd like to try making a play-in-browser game, where a 2nd player (and maybe more later as I figure things out), can just join a game in progress, drop out at any time with no penalty, and they can just play together without having to download/install, or buy something, etc.
Are there any guides out there on doing this, any existing play-in-browser games here like this, or am I stepping out into unexplored territory here?
What you're talking about is called client-server architecture, simply applied to browser games. It's been around for decades. It's not new or uncharted territory, but it's not very common, mainly due to performance issues.
I don't know any guides, but if you Google "Java client-server tutorial" or "Java socket tutorial," you'll find quite a bit of information.
However, Itch doesn't offer any server services for multiplayer games, so everything related to the server side (including the hardware) is up to you.
