https://bespin.mozilla.com/ - is a pretty slick in-browser code editor, that, if I'm not mistaken allows shared editing like Google Docs.
There is also an Eclipse plugin called Cola that allows simultaneous editing, but I'm not sure how stable it is, I've only seen a video demo.
And whichever method you choose, source control should be mandatory, ideally distributed (git or mercurial), though SVN is better than nothing.
Nature, to be commanded, must be obeyed. -- Francis Bacon