Initial team creation currently happens on the student side of GitHub Classroom. For the initial group assignment, if you want certain students to join certain teams you must specify the team names to your students and ask them to form those teams within the Classroom interface.
Once teams are formed, you may reuse them for subsequent assignments.
If you are having difficulty reusing existing teams, please report the issue here:
Please include a detailed account of the steps you’ve taken, including screenshots if possible.