On the physician directory, colleagues who are actively using DocbookMD are indicated by the blank, grey dialogue box on the right side of the screen. This indicates that you can send them a message.
If the dialogue bubble is not featured, then that end user hasn't registered for DocbookMD just yet. You can then click on their profile and choose to send them an invitation to join DocbookMD. Once they register, you can then exchange HIPAA compliant messages with that colleague.