Working with teammates through a NounTeam is a great way to simplify billing and keep track of the icons your team has paid for. To add Members to your NounTeam, you might have to check if you have Seats available to add a Member. For details on this read here.
Once you have available Seats to fill, go to your NounTeam account under the top navigation and select Seats. Then, on the right side, click "Members". You'll be taken to a new page where you can add a teammate by entering their name in the text field and clicking the plus button.