I can't remember (or find out) how to invite my elementary students (without e-mail addresses) to post information on my pbwiki site. I'd like to have several classes collaborate over several days time by adding comments and information to my wiki.
I just did this...with a pbwiki account for educators. It was free - but I listed that I wanted to do it for education -- and it allows you to add students -- and then it generates usernames and passwords for them. My fifth grade students are creating a USA wiki with each student adding a state page. They are really having fun with it! Good luck!