The first work to be done is to make a choice on the right framework or library that allows us to connect node.js to a database (which can be MySQL or can be something that fits better to node.js) and then a library for having a user management using node.js and that database and which could use the bitcoin address as username. If nothing like this already exists it would be the first piece to be developed.
How did you reach the design choice "node.js"? Why not rails, django, play or anything different? What do you see in node? I'm always interested in working on cool projects that bring new things to the world but I wouldn't want to without knowing anything. The basics you described can be easily implemented in any programming language however what is the goal?