The CP2K website now resides completely in a wiki. Most pages are publicly editable by anybody, just click the pencil on the right. To create a new page just open the URL (e.g. http://cp2k.org/my_new_page ) and click the pencil. The software we are using is called DokuWiki. It is very easy to learn. You can try it at this playground page.
Maybe you have already noticed the small “[Edit]” links at http://manual.cp2k.org/trunk/ . This is a new feature, which allows everybody to easily contribute to cp2k's documentation. The links lead to a form where you can submit text changes, after a short review the changes will appear in the official cp2k-manual.
So,…
The source code of CP2K is publicly available. You can follow the development and send us patches. Some helpful information for new developers is also available.