Your question may have already been answered, but I thought I would share with you what I did with my 150 students using my wiki. The day I introduced my classroom wiki, I had the students in each class period come to my desk and type in their e-mail address so they could receive the invitation. They then went back to their desks where we were using a class set of lap tops and opened their e-mail and accepted my invitation. I would also reccomend creating a "dummy e-mail" so that students with no e-mail or computer access can still use the wiki using the e-mail and password you created. I use wetpaint for my wiki and this method for inviting students worked well for me.
Think this is probably my second reply - can't find the first one to edit!
As I am a university teacher (in Spain) I don't think I would be eligible for the bulk user tool, it being only available to K12 teachers.