There are two ways to add users

  1. Share your custom sign up page with anyone who you want to be a user. (
  2. Bulk upload users with a CSV.  All added names will be added to the list of people who can be named in a story - before they accept the invite. But they are required to register before they can log in as a user. (https://your-custom-domain/company/settings/users) 
Tip Promoting User Sign Up: Add a link to your signup page in your intranet, a slack channel, internal newsletter, etc.