Eric, I assume that you have created an external custom page.
Buttons at the top of standard index page are part of the console ( when user is loged in same console show another set of features instead of sign in and sign up).
Since you are using external custom page, you will not be able to add core console functionality to it easily.
The only solution in your case, is creating your own .html for "Sign in" and "Sign up" buttons and add a link to /sign-in and /join pages.
Is there a specific reason why you have decided to use external page and not a local one?