This project was more or less closed as everyone associated with it dropped it.
However, I don't want to discourage any of you from working on it if you can find the motivation.
Do you know how git/github works? If so, you can send pull requests to
the official repo, if not just work locally (although I really /really/ recommend using git)
The Readme on github has a basic list of the dependencies but no instructions on installing them. pySFML and PIL have their own installation instructions, but the bitmask library is custom-built and has to be compiled yourself. The compilation is scripted in install.py, but you'll need MinGW if on windows (which you should have anyway). Tell me if you have any problems.
Jumping into an unfamiliar codebase is hard, I know. PyGG2 is structured so that the core game engine, the rendering and the client/server-specific parts are as separate as possible, and there are a lot of abstractions. I don't know your preferences, can't really recommend a specific part. Things to do are myriad, even very simple things like fixing the weapon sprite offsets as well as more difficult things like HUDs or adding classes/weapons.
If you have any questions, asking here will get them answered eventually, alternatively asking on the IRC (if I or someone else happens to be active) can get you answers faster.
Start by getting the game to run. That's (sadly) already somewhat of a hurdle, especially on windows.