When trying to sign up for a mailing list hosted at lists.ibiblio.org, there is a broken reCAPTCHA interaction which breaks the form. This makes signup without help from a mailing list owner or moderator impossible.
For lists that allow subscription via -request@lists.ibiblio.org, signup is still possible, so that’s one mitigation – but people have to know that the option exists!
The only global workaround (works for all mailing lists) right now is to visit the admin page for your list (assuming you are a list owner or moderator), and manually add or invite users from that interface. We are working on a long-term solution, but we do not yet have a timeline.