I agree with alphadog about using a wiki for collective documentation.
I've used MediaWiki for a number of projects, and recommend it highly. It's the Wikipedia software, from Wikipedia, bundled for the do-it-yourselfer, free to download and use.
Upload it to your web host, create a MySql database, update the MediaWiki config page with your database settings, browse the Installer page, answer a few questions, next thing you know it's your very own wiki.
The wiki makes it easy to record new ideas based on keywords, because wiki uses keywords as page names. You start with a Main Page, the home page. Make a few notes about your key ideas. Mark up the keywords as links by enclosing the keywords in double square braces. (There's a simple Wiki markup language, much simpler than HTML, the wiki program translate the markup into full HTML.) When you view the page, the marked-up keywords are now links. Click on a link, you go to the page by that name. If the page exists, it gets displayed; if the page doesn't yet exist, your browser displays the page editor, so you can get to work on the new page. Add more content, make more links. It's easy to build up content quickly, try some ideas, see where they go -- it's quick, encourages brainstorming creativity.
Meanwhile, other team members are editing existing pages, making new pages, adding and refining. Everybody can see and share the entire body of work, and there are various mechanisms for adding comments, annotations, etc.
Furthermore, the wiki keeps track of every single change from end to end, with notes about who did what, when they did it, and so on. Obviously for the real Wikipedia, you need this to revert vandalism. But even in a team effort where everyone plays nice, it's still a huge benefit to have a version control system keeping tracking, so you can view progress, retrieve long-deleted items that you wish you'd kept, etc.
If you want privacy -- a team-only wiki -- there a several things you can do:
(1) In the MediaWiki config:
A. Restrict page view to logged-in users, redirect all others to the Main Page (the home page for the wiki, which is always public).
B. Restrict editing to logged-in users.
C. Restrict account creation, no self-registration, admin must create new users
(2) HTTP username/password
Set the server to require a username and password for the folder where you installed MediaWiki. This is a server thing, not a MediaWiki thing as such. You can do this through the web host account control panel (for some web hosts anyway, I think it's common now; if you don't have that control panel feature, contact your web host).
When a user browses any URL in the protected folder/subfolders, the server requires a username and password -- if the login fails, the server doesn't send the page, instead the user gets a "Forbidden" message.
For more game design tips, see my game design blog, the Handy Vandal' Almanac: