Member -> This is anyone that gets invited is immediately given this role. They are essentially a student.
admin -> This is for Teachers, TA's, etc. It is meant for folks that will be creating assignments and grading.
billing admin -> This is for whoever creates the team and/or will take on the role of paying for the team.
This happens under Teammates. You will see a manage members link there which should open up a modal. In the modal next to each user you can assign the different roles.