You cannot email your Channel members at this time, but you can post a message in the Channel bulletin. However, you can email webinar attendees and registrants. You will need a paid plan to do this.
To access registrant and attendee email addresses, you'll need to go to your webinar page, and click "Promote & Manage." Within that section you will see "email attendees." There you can download an email list of those who have registered and attended your webinar.
If you have any questions regarding email lists or emailing in general, please contact firstname.lastname@example.org