How do you mean "shared" and "their domains can be used by others"?
Do you really mean that if I register example.afraid.org (for example) and point the DNS A record to 127.1.2.3, another user can come and point that same hostname to, e.g., 127.4.5.6 ?
Or do you "just" mean, another user can register example2.afraid.org, so the base domain is shared?