There are three ways an official can be added to your site:
Add an official individually from the People page,
Import an official's information from a spreadsheet, or
Share your Site Key with an official, and allow them to create an account on their own.
Your Site Key is a special URL that you can share as needed to allow officials to add themselves to your site.
You can find your Site Key by navigating to the Account page:
click your name, upper right corner of the screen
choose Account
You should then see an entry on this page labeled Site Key
Just share this special URL with officials, and they can use this URL to create a new Assignr account, or link an existing Assignr account with your site.
Per-Referee Plans
For sites on a per-referee plan, keep in mind that this signup page will only allow officials to create an account if you are at or below the limit of officials you have paid for on your plan. As an example, if you have purchased access for up to 100 officials, and you have 100 officials on your site, officials will not be able to use your Site Key to add themselves to your site.