To create a new teammate, go into Messaging (select your Airbnb account if needed) and go to the Teams tab.
Scroll a bit down the page to see the list of all teammates.
Add a teammate by clicking on the blue "Add teammate" button.
Please pay attention to the language that will be used to send a message to your teammate as this will define the template that will be used, and the format of dates and monetary values. In addition, please pay attention to the service/type, which will determine what type of message will be sent to that contact.