The style guide isn't secret.[1] Also, the research saying that most changes were done by a small group (~1500 way back in the day [2]) was challenged. While the core group of wikipedians are responsible for most of the copyediting, categorizing, etc, the bulk of material additions came from outsiders.[2]
[1]http://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style
[2]http://www.aaronsw.com/weblog/whowriteswikipedia