It’s not even in the menu. The only way to sign up is to tap “Log In” and then tap “Sign Up” on the Log In screen.
I filed a feature request about this on the Discourse forum.
But, in the meantime, I think there’s some stuff we could do to make this experience better.
There’s a “Discourse Combined Auth Button” theme component we can use to put a combined “Sign Up / Log In” button in the site header.
I experimented with this in Dev Tools; I think if we did this, we should make the mobile site logo just be the “IF” square, because “intfiction.org” becomes illegibly small with the text “Sign Up / Log In” in the button.