Feel free to scribble and vandalize. This board is less strictly moderated than the other demo boards. You can add the graffiti features (spraycan, nibs, colour selector, ink dropper) to your own Groupboard by ticking the "graffiti" option in the HTML generator.
To draw by yourself, use the Single-user version
Also check out Groupboard Streetart - draw graffiti on any street wall in the world!