If they've signed up to the Roll20 forums, you can PM them through the forum's inbox. If they're in the campaign already, you can pass messages back and forth via the campaign message board that you can access on the campaign's detail page (although your entire gaming group will be able to see your posts). Presently that's the only two ways to get in touch with your players. The developer team is currently working on a content update that's overhauling our forums and communication tools to make this process easier and more intuitive.
