From the event details page of an event for which you are the host, you'll see an admin option bar at the top.

To send emails to your guests, you can click the Mail icon (3rd from the right) as pictured below.

By default, all guests in the list (attending, declined, invited, waiting for RSVP) will be selected. If you'd like to send emails to specific guests, you can do so from the "Manage guests" screen.

If you click "Show", you can view and edit the list of recipients in case you want to make any last-minute changes.

The subject line is optional, and if not provided, it will be set to the title of the event.

The message field is an HTML-friendly way to send the body of the email.

Clicking "Send test to myself" will send a copy of the email to your email address, which will allow you to review the email before finally sending it out.