One of the classmates has been trying to join the site and is apparently using the correct email address and entered a password, but then when she clicks to go onto the site, Internet Explorer gives her an invalid address report. Any ideas how I can fix this? We have checked and the email address is correct.

Have you tried it and get the same error?

Have her try in a different browser (Firefox).

Sometimes these errors just go away by themselves.

No idea, but if I had to take a wild guess, the Classmate tried to log in, had a connection problem or something like that and got a dead page, but the page got cached. Now when she tries to log in again, rather than loading the next page fresh from the server it's loading the previously cached page. Try having her clear her cache (temporary Internet files), close down her browser, open it back up, and try logging in again.

No idea, but if I had to take a wild guess, the Classmate tried to log in, had a connection problem or something like that and got a dead page, but the page got cached. Now when she tries to log in again, rather than loading the next page fresh from the server it's loading the previously cached page. Try having her clear her cache (temporary Internet files), close down her browser, open it back up, and try logging in again.