Primary Pad is so simple and requires no sign up, registration or the need to invite students via email address.
Basically, it's a site that enables you to create an online collaborative document, similar to google docs, but it's quick and easy. with a free account, you can have up to 15 collaborators and your document will remain for 30 days then be erased, but you're able to export it as a pdf.
It may be tricky having more than 15 people working on the same thing at once anyway.
The best part is you can create one in 5 seconds before your class starts!!
Type the name you want for your document after the web address http://www.primarypad.com/
so for eg. I want to create a document on the history of Jazz, so i type http://www.primarypad.com/jazzhistory and if nobody has made a document of that name, it will just ask if you want to create it and voila! There it is! All you do then is share the link with your students and they can jump on and contribute. Each user has a different colour behind the text so you can see who's writing what. There's a chat feature to the side of the document too which allows them to discuss ideas before editing the document, but I can see this being a little distracting for some students...Keep in mind that starting a document this way without creating an account will make the document public.
Teachers may wish to use this for things such as meeting notes, minutes, group planning etc.