While Bookwhen doesn't offer a feature to send emails from within the system, you do have the option to copy your attendee's email addresses easily into your own email system or to launch your email tool from the attendee list.
To do so, navigate to the Attendee list for your event:
Then click the Options drop-down, and click View email list:
You can then use the option Open in your email client to automatically open a new blank email (BCC) in your default email tool on your device:
Or you can use Copy emails to easily copy your email list and BCC them into your preferred email tool manually:
Here's an example of how that can look, using Gmail: