This is straightforward; just sign up!
To log your calls, you need to grab one of the following:
Once you've added it to Chrome or Firefox, you're good to go! Just log into GrassrootsPB, use one of the linked calling tools from the front page as usual, and start calling! Note that you must have the Chrome/Firefox extension installed before you log into most calling tools.
You can click on the "PB" extension icon (near your address bar) and check your current stats in the "popup" that shows up. The extension's icon will have an "!" overlaid if you're not logged in to GrassrootsPB - so pay attention, or else your calls won't be counted! Note that you will have to hit the "refresh" button in the top right corner of the extension in order for your stats to be updated.