Neither documentation nor the tutorial you're following mentions that you need to rename package.json.sample and Gruntfile.js.sample to package.json and Gruntfile.js respectively. Rename these two files and run npm install and npm update.
Make sure you have grunt installed globally as stated in the documentation/tutorial. Don't forget to remove your previously installed package.json and node_modules.
Please read the documentation: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-topics/css_debug.html
grunt exec:theme NOT grunt:exec
