I just had someone sign up for my discourse while on the phone with them, and she could not find the sign up button. I hadn’t realized this before, but on narrow screens (eg on mobile phones held vertically) the sign up button is not shown next to the log in button. When the phone is held horizontally, the sign up button appears again.
The sign up link is displayed on the login screen, but she did not get that far and says she would have given up. She selected the hamburger menu looking for the sign up button, though, and made the UX suggestion that it would be helpful to have the sign up and login buttons displayed at the top of the sidebar menu.
Looks like it’s been like this for a long time!