How can we organize our file-system and processes if there are multiple publishers on an npm module? Do we need a common repository (ex. GIT) or is there a smart way to use npm's own publishing & updating process?
The main issue I can't get my head around is that the initial publisher of the package is not able to get the latest version from within the package itself, is he? Unless he installs it as a dependency on another package and then updates & publishes from within that dependency.
It's certainly possible to use npm in this way, although it's probably not what it's intended for. The initial publisher of the package will create the necessary structure, e.g. using npm init, then do an npm publish to make it available to your other publishers.
A couple of things to consider. First, unless you have a tightly controlled system for taking it in turns to edit, you'll almost certainly get merge conflicts, where multiple publishers make changes simultaneously. A version control system like Git can help resolve these conflicts.
Secondly, you may not want your intermediate versions to be publicly available, for many reasons - it's likely you'll want to push out some (possibly incomplete) changes for your co-publishers to build on. Or you just might not want your code to be out in the wild during development. So you may wish to consider a private repository if you do go down this route - e.g. Sinopia, or one of the hosted solutions.
Hope that helps. For info, I use a combination of mercurial (for version control) and a private npm repo (sinopia).