There are a few handy tools out there that I consider to be game-changers.
Etherpad is one of them.
According to http://etherpad.org/ it's an open source project that is designed to allow you to edit documents collaboratively in real-time. You can also add in tons of plugins that allow your instance to suit your needs.
You also can run it online from one of the public repositories.
This little tool has a lot going for it and it's just simply fantastic. It is so simple to set up, so simple to use, and has great functions. It's ideal for a group collaboration and doesn't require the added fluff of Google Accounts etc. (although Google accounts do have their own merit, this is a sure-fire way to get a simple solution up and running quickly and is more like a use-once-and-then-burn option).
or check out my live demo here, and say hi :)
or, check out a very good and populated demo here with some extra features: