Yeah, sorry about that. An unfortunate part of making a multiplayer game. You could try asking in the BlackthornProd Discord server if anyone wants to join you in a playtest.
As for how I made it, it's actually made in javascript using the Phaser engine and a library called PeerJS for the networking. I didn't need to build anything since javascript is an interpreted language, not a compiled language. For testing, I just changed it to boot directly into the game and most of the testing could be done with a single person. If I needed to test something related to multiplayer, I'd just boot to the menu instead of the game and open a second browser to join myself.