New teachers can sign up for Freckle on their own by going to https://classroom.freckle.com/#signup. Once they choose their school affiliation and verify their e-mail, they will be automatically added to your school account, and you will see their data on your Administrator Dashboard.
To personally invite new teachers to join Freckle, administrators can go to the Manage Teachers page on the Administrator Dashboard. At the bottom right of the page is a place to invite new teachers using their school e-mail address. They will receive an e-mail notifying them of your invitation.