And my new yahoo account can't join any groups :/
> Your email address is not linked to a Yahoo ID. To join this group, you need to link your email address to a Yahoo account.
Then you need to go into "Account security" and link an email address with the account. I don't know why the instructions don't tell you this.
Thanks for the suggestion though, I saw that thread about linking a fallback email address as well but that seems to be a different thing.