Here's how to add multiple recipients to a new message:
Select +Add New → Message in the left-hand menu.
Select a Recipient (Contact) and associated Account in the To field.
Select OK.
After the recipient is added, repeat steps 2 & 3 for each recipient that should be added to the message.
Here's how to add new recipients to an existing message thread:
Select Additional Recipients in the Reply box at the bottom of the message thread.
Select the desired Recipient and Account.
To add more recipients, repeat steps 1 and 2 as needed.
Enter and send your message to add the Additional Recipients!
Using the Additional Recipients option is how you can "forward" a message to a coworker or another user.
Any additional recipient you add will be able to see the entire message thread and files associated.