Using wikis for collaborative writing
Most of the wikis cannot handle conflicting edits, aka. two people writing the same page or chapter at the same time. The first who gets to save his edit, gets it through, but those who started editing before the first editor did his last save are greeted with an announcement that the page has changed since they loaded it and depending on wiki gives suggestions on what to do next. These situations are always quite confusing and easily get to be complicated, so with students we'd like to have as few conflicting edits as possible.
One way to avoid conflicts is to spread the edited text into smaller chunks and for every chunk have their own page or pages and designated groups of pupils working on those pages.
Another hint for avoiding conflicts is to remind pupils to keep making small changes but making them often. Longer it takes to write changes, the more probable it is that someone has already edited the same page while writing.
If a pupil wants to write a longer chapter to a conflict-prone page, it is reasonable to start her own page and write it first there, and then quickly copy-paste it to its final place.
Linking between pages and keeping the structure of network of pages in some orderly manneri is often the part that causes most of the technical difficulties: if link names differ from page names even one letter, a link can lead to a nonexisting, or 'new' page. Teacher's role in collaborative writing should be organizing the network of texts and suggest good subjects for pupils.
WIKI is a perfect tool to use for collaborative writing. You as a teacher just think about a topic to write about. Then create WIKI starting page and links to new pages. The easiest tool to use is PBWIKI. Then explain the task to your students, tell them the address of the created WIKI. Students just write on the assigned topic, add pictures, links and word explanations if necessary.