For anyone else trying to add grunt, you need to copy over the gruntfile.js and add the grunt dependencies to the dev section of package.json in the root directory of the project. Then you might also have to remove and reinstall the grunt nodemon package (grunt threw up instructions on how to do this when I first ran it)
https://github.com/linnovate/mean/blob/master/gruntfile.js
Make sure you have grunt installed.
http://gruntjs.com/