Can you pull the emails collected through a "form" and dump them into a Civi CRM group?
Welcome to SE!  There are several ways to add email addresses to a mailing list. See this section in the User Guide.
What CMS are you using?  On Drupal, you can use webforms.  On WordPress you can use Caldera Forms or Contact Form 7
The GDPR extension also provides a page for people to choose which groups they want to belong to.