Why do people see the message "Sign-up not allowed"?

If you see the message Sign-up not allowed, you have probably tried to gain access to a team portal from an email address that hasn't been whitelisted. 

Here's the model URL: https://{customdomain}.start.me/users/sign_up. What you see is this screen:

When you click Request access you, you either gain immediate access to the team portal, or you see the following message:

Send a message to the team owner and either ask that person to invite you personally, or to whitelist your company email domain. Note: when you whitelist an email domain (e.g., @mycompany.com) everyone with the same domain automatically access to your domain.