To work with pygrass you need a up-to-date version of GRASS 7. You can obtain a recent version following the information on the main web site of GRASS, and you can read more about compilation on the GRASS wiki
Now you can download the pygrass source using git
git clone https://lucadeluge@code.google.com/p/pygrass/
If you have not git you can install it from the git website or download the source code of pygrass from http://code.google.com/p/pygrass/downloads/list
At this point you have to install pygrass, so enter in the right folder and launch with administration permission
python setup.py install
The last action before start to work with pygrass is to run GRASS 7 and from the console launch python or ipython (the second one is the the suggested)