Private messages on CareSpace work in a similar way to Facebook. Firstly you need to add the person you want to message to your friends list. You can do this by clicking on their user name - this will take you to their profile page. Once there, click on "add as friend" in the top left of the screen. If they approve your friend request you will be able to send them private messages by clicking "start a conversation", also on the top left of the profile page.
CareSpace support