The backend is a nodejs application, so to start it, you need an installation of nodejs and npm. Then you navigate to the crosswatch_backend folder, and it's just
npm install
to install node package dependencies for the server, and
nodejs server.js
to start the server.
By default, the server runs on port 3000. It reads the PORT environment variable, so you can set a different port by, for example
PORT=4000 nodejs server.js
The port needs to be open, and participants need to set the server location (and port) in the extension options.
At the moment, I am not running a server meant for public use, so you will have to sort this out with the group that you're watching with.