Adding students is easy and you can add them whenever you need to.
To do this, log in to your teacher account and go to the "Roster" section. Below the list of students, there is a button "Add student" that you need to click. You can also find more information on how you can edit your class roster HERE.
Student Roster size is limited to 35 students for the Basic Classroom license and 75 students for the Advanced Classroom license. To learn more about the differences between license types and what features are available, please visit this page.