You can send a private message via email to a specific Group member. Click the plus sign next to Group Stream, then click +PM.
Select which group member to send the message to, then compose your message.
Click Enable formatting to bring up the full WYSIWYG editor if you'd like more formatting options.
Note that private messages are not logged anywhere in CoachAccountable. If you'd like to send a message that's part of the client's record, instead go to that client's page and click the email icon, checking the box for "Add this message to Stream" before sending.