I have a pretty straight-forward question. I made a web game with NodeJS, and I can successfully play it by myself with multiple browser windows open side-by-side; however,
The port is probably blocked by your local firewall or router. Hard to tell without details.
But there is a simple solution for which you don't have to mess with firewall rules, run node as a privileded process to serve on port 80, etc...
Check out Localtunnel. Its a great Ruby script/service, which allows you to make any local port available on the internet within seconds. It's certainly not useful for a production setup, but to try out a game with colleagues, it should work just fine!