Domain sign-up allows you to easily add team members by pre-authorizing your email domain, so that anyone with an email address from that domain is automatically added to your team when they sign up (for example: your website is www.grizzlybearcorp.com and goldilocks@grizzlybearcorp.com could sign up automatically).
You can turn this feature on or off at any time from your account. However, please be aware that commonly shared domains (i.e. gmail.com, hotmail.com) are not eligible for domain sign-up.