To send a Private Message to any number of your ACC Buddies, click My Buddies on the navigation menu. Then click the small square check box next to each Buddy you want to include in the Private Message. Finally, click Invite Selected Users to Private Thread.
You can also send a Private Message to anyone by clicking the little envelope icon in the top right of a message they posted on the message board.
After that, you just Post a Message like you normally would. The people receiving the message will see a Private Thread alert in the navigation menu to let them know they have a private thread waiting.