You do not add students to an organization directly. 

They are added by inviting them to join a course. See Add Students for how you do this once you have created your course
and then Joining a course as a student for how your students then join where they either click the Course Invite URL you give them and sign up for an account with Codio where they will then automatically be added into your Organisation and the Course or they go to https://codio.com/p/join-class where they manually enter in the 2 word token and sign up.

The Course Invite URL is the better option as there is less chance that they can mistype the 2 word token