Did they select a language? Mine did the same thing on various instances. I tried a few things at the same time, so I don't know what finally let me register, but I learned a few things in the process.
For example, the most likely explanation is that the username is already taken. The back end fails silently when that happens, and there's already a bug open in GitHub. I figured I'd try a shorter password and selecting a language, and that worked.
Hopefully that helps your friends.