If you were invited to create a free account and cannot locate your invitation email, it can occasionally be sent to your Junk or Spam folder, or in the case of Gmail, the "Promotions" or "Social" tabs. Doing a search using notifications@corsava.com will often locate the email. 

This can also be the case for your students, if you invited them via the email process in your account, so remind them to take a look in these folders as well.