GoogleDocs are much more effective for group work than wikis because everyone can edit them at the same time. I would suggest placing text in a table with cells to make it easier for students to decide where to begin typing - or have them create some type of outline of heading for organization before collaborating. The main drawback is that you need to be 13 to have your own Google account.
Do you have Google Docs for Education at your school. If not you should consider asking for it. You can customize everyone with a the same email domain, doc, sites, calendar, chat (I don't use this). Its easy for my students to share docs with each other because all our email addresses are the same email@example.com. I also use the calendar for due dates and place directions and handouts on Sites. I administer all of the accounts so I have full control over who has access.
If you decide to go with a wiki (I use Wikispaces - free for teachers!) make sure you make plenty of pages for students to write their own information. Most wikis can only be edited one at a time.
I have just started a interdisciplinary project using a wiki to connect students BUT I am encouraging them to create the product, a PowerPoint presentation, with Google docs. Not sure how it will work but so far I am pleased with the wiki as a networking tool.
Google Apps for Education (www.google.com/a/edu) includes google docs as well as google sites. I think that sites is a much better option than docs. It allows for multiple editors at the same time and much more variety in formatting options than google docs.
You could also try wikispaces.com They are very helpful in setting up for educators.You can only have one editor at a time though.
Yes you do have to invite each student individually. Otherwise you can open it up for public access. Once the students are signed up it's really easy... you simply start typing their name and names will start appearing in a drop down list (just like in gmail to: field when composing an email).
You can keep track of site changes by clicking 'More actions' at the top of the page, then 'Subscribe to site changes'. Any changes are then emailed to you.
I tested google docs with my under 13 crowd and found it easy for them to create their own documents that I was not invited to which made me uncomfortable to say the least! I now use Writeboards. They are free, don't require an email address, and you can subscribe to them via RSS, something you can't do with a google doc. Writeboards don't have all of the features of a google doc but do let you see all of the changes made. They have horrendous urls so I link the writeboards to my wiki where kids and families can find them easily. I haven't tried etherpad but plan to test that out, it looks really neat!