It does seem like an interesting experiment, but after browsing a few random commits - and personally, any group project needs an initial structure for it to work. I can't remember right now but I have a faint memory of the subject somewhere, probably in college.

I would go over to http://creativecommons.org/choose/ and pick the least restrictive license or which ever works for the group / project.

You could integrate it with gitbook as fendent mentioned (I have no experience with gitbook past the homepage), or leanpub. I personally use leanpub which integrates nicely with github and the royalty fee is sweeter when compared - despite whether it will be the goal of the group or not. Or the funds can be donated, whatever gets the group motivated or is in line with the group. However, you do need to provide some sort of direction to the group. But as you mentioned, if it's just an experiment (the replies here are in itself data) go for it.

If you do decide to give it structure, you can take a look at a very basic book I set up a while back.

https://github.com/teichopsia-/valse_of_silence There are others out there that are better but I don't know of any.