You can send a "PM" or private message a couple of different ways. If you click on the name of the person, you'll be taken to their profile. On the right side of the screen, you should see "send this user a Private Message".
Click it and you'll be taken to the PM window with their name already in the To: field. Type up what you want to say, hit send message and you're good.
You can also go to your own profile by clicking on your name from the forums and then clicking the PM icon. It's the little envelope icon in this picture
From the PM window select "New Message" and type the user name of the Boost Community member in the "To" field and off you go.