I have been under the impression that writing is mostly a solitary activity, but that just points out the fact that I never thought deeply about the subject. Collaborative writing is everywhere, from a small newsletter to a major newspaper, paper and e-publications, projects, grants, numerous other applications.
Anticipating a collaborative writing project seems about equally exciting and intimidating. Since I've never done it and am only just learning about it, it's pretty much a mystery. The introduction is quite informative, though, and is reassuring. Thank goodness I know a little about Google Docs, just a little about wikis and am familiar with video conference calls.
Working with other writers would be exciting and rewarding, I think. They can critique your work and let you know if you are writing well enough to optimize the group's outcome.
I think WPA Outcomes is entirely about encouraging and teaching individuals to write better and guiding the teachers who teach them. The result is a world that is more capable of communication which leads to an uplifting of the whole, and that is in itself a collaborative process.