Go to Dashboard >> Candidate Search.
You can select the ones you want to send the message to, and click the Actions icon >> Send Message.
You can also use the Select All Candidates option (tick box on the Header row) and click the Actions icon >> Send Message. This will send the message to all the candidates on this page. You can also increase the number of candidates to be shown on the screen (up to 100).
You can also use any of the filter options at the top, or by clicking on the More option to filter your search.
And select the candidates you want to send the message to from the search result and then click the Action icon >> Send Message.
The Send message to pool candidates window will pop up. Type the message and click Confirm to send it. This option only has the Mail option.