You can grow or build your email list by encouraging your website visitors/users and customers to share their email addresses with you. You can provide them with dynamic, valuable content and encourage them to subscribe to receive email notifications from you.
Tell them about the free plans/offers that they can enjoy by opting-in to receive email communications from you. You can use email marketing software to create a signup/subscription form and share the same on your website, landing page, social media accounts, etc.