We here at DocbookMD understand that our end users would like to send a single message to multiple recipients. Currently, there is no way to send a group message. Group messaging is definitely on our road map and this feature will be added within the next couple months.
As a work around - For now, you can try forwarding the message.
After you send a message to one recipient, revisit the same message and click the arrow icon at the bottom of your display.
You can then forward the message to another recipient.
We understand that this work-around is not ideal, and can be tedious, but it's currently a way for you to send one message to multiple users. We apologize for the inconvenience.