You need to add candidates to a campaign to start messaging them. You'll need to have an email addresses for the candidates you want to reach, so you will need to navigate to your Projects and get their contact information first.
Once you have email addresses for your candidates, select the candidates you want to add to the messaging campaign and click Add to Campaign under the More menu.
You can also add an entire project to a campaign. Navigate to your project, click on More in the top-right corner, and select "Add All to Messaging Campaign".
Any candidates you add to a started campaign will begin receiving the first message after about 20 minutes. (We have a small delay in case you accidentally add the wrong person to a campaign and want to remove them.)