If a contact is not opted in for email marketing, and they wish to be, you can add them a couple of different ways. From your site, where you collect emails, or from within Shopify.
There are two methods available for individuals to subscribe to your email marketing.
Via your store website
Enter email in Subscribe to our emails
Within your Shopify dashboard
Go to Customers tab
Click on customer name
Click three dots in Customer section
Edit Marketing Settings
Check Customer agreed to receive marketing emails.
