As an adult student (over 18 years old) you must receive an invitation from your teacher in order to sign up for WeSchool and access your Class Group.
If you are looking for help, you can follow this tutorial about how to join a Group whether you have received an email invitation, an invitation link or a class code.
From the registration page, click on "I’m a Student":
Then fill out the form with your personal data, accept WeSchool’s Terms and Conditions (the first check is mandatory, the second and third ones are optional) and click on REGISTER: