Sometimes, it may be desirable to hide the "Sign up" button for the Portal:
Initially, I thought the file to modify would be in the portal2 folder. Specifically portal2/portal.js but it turns out what's being called here is clients/portal/views/login/login.php
As always we should never modify core files. So you can make a module loadable package for custom/clients/portal/views/login/login.php and comment out lines 23 through 28 that talk about the signup button.
Attached is a module loadable package for all 8.x instances to do just that. Please note 9.0 will be out later this year, so the manifest.php may eventually need to be modified again.
Big thanks goes out to Kevin Louie for helping with this.