Once you created your classroom, you may want to enable the URL based inscription to avoid the hasle of registring all the trainees yourself.
This option is located in the options panel of the classroom edition.
Once this option is enable and saved, an URL will appear below the checkbox. You can then copy this URL and send it to whoever you want, or post it wherever you want. When people will follow this URL, they will be able to login or register and will be automatically registered in your classroom.
Please note that this operation will allow anyone who is in possession of the URL to register, and thus may lead to unwanted peoples subscribing.