If you want to email your event’s attendees you can do it in two different ways:
Login to your collective, go to the event, and under the title of the section for all registered people you should see this options:
By sending an email to $event@$collective.opencollective.com to reach out to all people who RSVPed.
To email all your backers you can send an email to backers@$collective.opencollective.com
Another option is to login to your collective and head to the menu right before the expenses and budget section and click on the “EXPORT MEMBERS.CSV” link.