Can I see if my students have joined their class for the app PLAY?

You can find an overview of your classes by clicking on 'Classes' in the menu on the left.

Click on the name of each class. Here you'll find the names and details of all the students who have already joined. At the bottom of the page you'll see the students who you have invited to the class but haven't yet joined. 

To create a user account they only have to provide a user name and a password. You may want to ask them to use their names or provide usernames you can easily identify.