Dropbox and edit directly from there. This means that any changes made will be synced across all computers working with the relevant Dropbox folder. Also useful if you want to edit your own work from multiple computers, of course!
Word of warning: Dropbox can't really handle having multiple people editing the same file at the same time. Their solution is to create a copy of the file to ensure that conflicting edits are not lost, and there doesn't appear to be any way to merge these documents afterwards. It's probably best to stick to something like Google Docs (or Drive as they now call it) if you want to live brainstorm or are likely to need to work on the same project simultaneously.
If anyone can suggest alternatives that would facilitate version control I'd love to hear about them.