Yes, you can send direct messages on the Langmuir Systems Forum. While signed in, click on either another user's username or their user icon. In the pop-up dialogue box, click the option that says "Message".
To keep track of your direct message conversation and see replies, navigate to your user page by clicking on your own user icon in the upper right-hand corner of the window. Then, click on the "Messages" header near the top of the page.