To enter your invitation code, go to stileapp.com/invite and enter it there or click this button on stileapp.com:
Alternatively, if you're already logged in, you can also use this button at the bottom of your subject list:
My code has expired!
You only need your invitation code once. After that, you're permanently enrolled in the class. Simply log into Stile and the class will be there.
If you tried joining a class for the first time and the code is expired, you'll have to ask your teacher for a new code, as they expire after a week.