After some quick research I realized, from the grunt documentation, that grunt has to be installed in the project. I did that by running:
sudo npm install grunt --save-dev
Now grunt works.
I also needed to run:
npm install
so that all the dependencies are loaded.
This solved my problem. To be honest I thought yeoman would have taken care of the last two steps automatically.
In case you have comments or think there is a better solution, please leave a comment.