*Addendum: The invitation link isn’t needed anymore to join Class Blogs. Instead, click on the Join Here link on the homepage. However, you can still use it from your Class Blog to invite students or teachers to join your site.*
When I sent out the new invitations, I did not realize that the code was reset when I removed Buddypress from the system. So, in order to make the link work, you will need to change the link to something like below making sure to add your email address at the end. This link bypasses the need to click on a signup link, so this link will get you into the registration page and you should see the registration form when you get to this page. Sorry for the inconvenience.
http://classblogs.us/signup-898/?action=register&email=myemail@gmail.com