You can manually add a user to your storefront at any time and grant them access to content. Doing so will no longer require them to sign up.
Navigate to the People tab and click +Invite People. You'll need to enter their Name and Email Address and you'll see an email template below. You can edit the email further to personalize it, but make sure you don't remove any of the #{{variables}} as those are needed to provide the user with their login information.
NOTE: If you want to invite multiple users at once via spreadsheet, follow these instructions.