Anyone with school admin access or higher can add new staff accounts from the school settings page. That will display a list of all of the users at your school or district broken down by current access level.
You can use the "Add" button to create a new account by entering in one or more email addresses. Those users will receive an email letting them know you've invited them to create an ST Math account. When they click on the link, they will be prompted to create an ST Math password.