I understand how someone can miss the "Login or Signup" tab in the moderator bar, but how does anyone miss the huge SignUp Now button on the right sidebar? If they do miss it, I can't imaging what reasonable adjustment you can make to help.
Two suggestions if you feel your site needs more.
- Add XenFans Welcome add-on. that provides an additional sign up button (highlighted in blue in the image below).
- Enable your "Right Content Area" block in the ACP. That offers another large display where you can ask readers to sign up