If your students already have member accounts on The Problem Site, you will need to use this feature to allow them to join your Virtual Classroom.  Enter a password or password phrase in the textbox, or click "get random code" to have the site select a code for you.

Now click submit to store your Teacher Code.  Note that the site will append a number (your teacher ID number) to the code, in order to guarantee that there are no collisions with other teachers who have used the same code.

Only give this code to your students, since it allows anyone who knows the code to join your Virtual Classroom.