I've noticed that yo angular generates a GruntFile containing all of the instructions, but typically does not include grunt.loadNpmTasks('<package>') statements, which may or may not be indicative of globally installed packages (?).
So, what is the correct way to add additional NPM packages to a yo-angular generated project?
the yo angular generator uses load-grunt-tasks, which prevents the gruntfile from becoming to bloated:
This module will read the dependencies/devDependencies/peerDependencies in your package.json and load grunt tasks that match the provided patterns.
the generator uses load-grunt-tasks without a pattern provided
require('load-grunt-tasks')(grunt);
which means that it will simply try to load everything (with the pattern grunt-*, for more info see comments below) you install via npm.
long story short, just npm install it (grunt tasks should be installed with the --save-dev option, to automatically move it to the devDependencies)
$ npm install your-grunt-task --save-dev