You will get a warning that the email of one user cannot be added. That will be the Demo Student. You do not need to worry about this.
If you click the Address Book button you should be able to use the Filter By: dropdown to select your class. (This may take a bit to load.) Select the checkboxes for the users you wish to email, then click the To, Cc, or Bcc to fill in those fields. Click the Add Recipients button at the bottom. Complete your email Subject and Body text and click Send.