Some useful links:
how to implement login auth in node.js
creating registration and login form in node.js and mongodb
Also session management isn’t done by Mongoose, but by connect-mongodb or connect-redis. You can checkout an example on how to do user auth and session management in a real application here:
https://github.com/alexyoung/nodepad/blob/master/app.js
Further explanations for that app you can find here: http://dailyjs.com/tag/lmawa or http://dailyjs.com/2010/12/06/node-tutorial-5/