Here's how to add multiple recipients to a new message:
Select +Add New → Message in the left-hand menu.
In the Message to field, search for a name, account, or email address.
Select each recipient you want to add - they'll appear as removable chips at the top of the field.
Repeat step 2 for as many recipients as you need, then select Done.
Here's how to add new recipients to an existing message thread:
Click into the Recipients box above the Reply box.
Select the desired Recipient and Account.
Repeat as needed.
Select Done and send your message.
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.