You can message applicants via the Manage Applications page.
To batch message candidates, use the tick boxes to select each application and then the drop down 'With selected' options at the top of the list to send a message to all.
You can also use this page to send a rejection message through the site. You can do this individually or in batch by selecting the candidates and using the 'with selected' options to reject a group in one go. When sending a rejection, you'll have the option to add your own message before sending.