You can connect WebinarNinja to a number of email marketing services such as Drip, HubSpot, SendPulse, Seva (formerly ConvertKit), and many others with Zapier. To know more about Zapier, please see this article from Zapier's learning center.
The integration works to automatically add webinar registrants to your mailing list. Do this if you want to continue communicating with your registrants after a webinar or collect their email addresses for future use.
Note: Existing registrants will not get added, only those who signed up after the integration is activated. To add existing registrants, export the list as .csv then import the file to your CRM system.