Brad,
Here's a response from someone trying to register, "Home page says to create new account go to Classmate Profiles and click on my name. When I click on Classmate Profiles I don't get Classmate Profiles, but instead get the You have reached a password protected . . ." page, from which this email is sent.
Please send me a system password or at least further instructions. Thanks!"

Any suggestions for helping this classmate get registered?

From this message, it tells me that you have password protected your "Classmate Profiles" page. This means that the classmates that are trying to sign up will need the "System Password" (create the system password on the bottom of the Edit Site Pages screen) in order to even sign up for your website.

You can make the "Classmate Profiles" page not password protected (Edit Site Pages screen, uncheck the check-box in the Password column next to Classmate Profiles) OR you can just reply to the e-mail (and every e-mail of a classmate trying to join) with the system password.