The sign-up reward is a reward setting designed to credit points to customers who have newly signed-up on the online store.
The sign-up reward is also available to customers who have been added from the CSV import feature.
Customers who have been added to the store manually or by CSV import possesses a Shopify account but do not have an activated account on the online store. (The customer page will be displayed with "No classic account". )
You can click on the "more actions" drop down on the upper right corner of the screen and click "Send account invite" so that the customer can create an account on the online website.
Once the customer account has been activated on the online store, they become eligible for the sign-up reward.