You can send multiple invitations out at once via the "Invite Candidate" dialog. Simply click the link, found in the top right menu bar of the application.
From here, you can invite multiple candidates by adding each into the email address field. If you have a list of emails, you can copy and paste the entire list (either comma or line separated) and the dialog will pick up each email.
Each invitation will be sent individually, not as part of a group email. If any of the invitations fail to send, the others will still be sent.