We automatically send your attendees a confirmation email and a 10-minute reminder (both of which can be disabled from Edit Event > Advanced).
You can also send a manual email to everyone registered for your event by following these steps!
*Note that the subject line will read "Message From ::your name here:: via Crowdcast"
While you're in the event, click on the Options button that's found on the top right corner:
A drop down menu will appear and give you the option to Email Attendees:
Once you click on that, a pop-up will appear where you can begin typing your email: