Subtitle for this would be "Legitimizing the geeky things I did in high school."
Collaborative writing spans many things, from the group of elders at a local church who work on their memoirs together to the group of romance authors who beta read each others' work to the online role-plays popular among many high school and college age people.
It's a great tool; collaboration with others encourages you to write more, both in terms of wanting to keep up with other people's output and encouragement in that other people are reading your work. And the immediate feedback helps you improve as a writer.
One of my current projects is collaborative; I'm writing a story with a friend of mine. He loves the world-building side, and I really like storytelling, so we work well together. I do a lot of the writing, posing him questions as we go along to be sure it's a coherent world. He goes over what I've written, tweaking some things that stand out and adding more to the story, especially focusing on fleshing out the character that is primarily his. And, most helpfully and most fun, we get to discuss it. The story becomes that much more concrete with someone to discuss it with.
That is one of the perks of a critique group, another form of collaborative writing. You may not, and probably don't, write in each other's stories the way I and my friend do, but the feedback helps you shape the edits, and having a group to discuss it with helps make the story more real; it isn't just something you are doing, it is real, and other people are reading it. I find that for me, knowing that I have an audience helps me remember to fully articulate my points in my writing.
A role-play is an even more involved collaboration; rather than it being one person's story being examined, or a project with an agreed-upon outline, most are free-form, with the plot developing as it progresses. In general, every writer writes from the point of view of their one or more characters, and have little or no understanding of the other characters except as they are presented in the role-play by the other writers involved. And the number of contributing writers can vary from two to over thirty, such as in some of the more long-running ones on the popular site LiveJournal.
The image of writer as a solitary creature chained to typewriter, emerging only for coffee, doesn't necessarily hold true. There have always been support networks, artistic enclaves, to further a creative spirit. Now there are more, more readily accessible, and more tailored. The friend I am collaborating with lives 2000 miles away; the memoir group at the local church all meet in person, and all have similar interests. If you are interested in a support network or a more collaborative sort of collaborative writing, a quick google search will most likely find you something that fits your needs.