Log in to your account, navigate to the Manage Box screen, and find the Invite via email link in the Box Launch Checklist box on the righthand side of the page. Click this link to open a new window where you can enter your name or your organization's name (required), an optional message, and the email address or addresses of those you would like to invite to provide feedback.
If you prefer to send emails through your own email client, simply copy and paste the text that is presented in the new window.