Some people's numbers may have short codes blocked by their wireless carrier (more often with Sprint/T-Mobile than other carriers), and receive a "Service access denied" message when they text your number.
As a result, these individuals will not be able to join your list through a short-code keyword or website form. They can either contact their wireless carrier and enable short-code texting, or you can import them into your account. Or, if you have a long code, you can contact them using that number.