Iris commented on my post below about use of Writely to share a document. In this case it was a document started in Brazil and finished in Portugal. What a great idea!
From what I can see so far, students who were going to be working in Writely as individuals would need only an email address and password. It would be easy for the teacher to set this up by posting a document and then inviting students to collaborate on it. The teacher would invite the students by providing their email addresses. They then log on with a password that they are given in the invitation email. That password can, of course, be changed.
I can't see how it would work to have more than one student editing a document at a time because I have to log out of one of my accounts to log in with another, but I think it should work well.
I really want to try this in the coming semester. Thanks, Iris, for the encouragement and example!