Per the GDPR and email marketing best practices, building an audience requires the full consent of each and every subscriber. This means that contacts should only be in your audience if they personally signed up for it.
This is a good thing for businesses. People who subscribe to your marketing are already more engaged and invested than those who could be added to an audience through less consensual means. However, it’s important to retain that initial interest and build upon it over time.
Subscriber data is usually gathered via signup forms, often embedded in a website. However, the ways in which these forms are presented and brought to customer attention can make all the difference when building an audience.