I found this comment from Josh Hug in another blog post.
We actually just pushed an major update to our invitation system last night. Our small team has been slammed just trying to keep our site running recently. I apologize for not pushing the team to address usability complaints about our invitation system earlier.
Tim from LibraryThing has pointed out some aspects of Shelfari that were definitely broken and are now fixed. We’ll be blogging more details soon.
Can someone confirm that the system is fixed now? I deleted my account, so I cannot.