How can I mark a customer as 'subscribed?'

From the Web

The only way to have a customer automatically 'subscribed' to marketing, you must be using our  subscribe widget on your website. The customer must enter their email (or first, last and email) into the form widget.

From the POS

To subscribe a customer from the POS, there is a drop-down option for 'Marketing Status' when entering the customer details;

From the Admin

You can also subscribe a customer after their purchase by clicking on Customers > Click on the customer you want to edit > Click the pencil icon next to their name > Choose subscribe from the drop-down called Email Marketing Status.