Just in case anyone else is wondering...
The answer was provided to me in a separate email, essentially its the same "custom message" box available when you're doing single invites.
Official response here:
Are you referring to the option to customize the Invitation message? If yes, you will still have the opportunity to edit the Invitation message after hitting the Add Invitees button. It does not send the invitation right away but it will bring you back to the previous page as shown in the sample below: