Just like you can link your GR account to Facebook, you can link it to Twitter. If someone chooses to enable that without giving any other name info, the system uses that Twitter string to identify them. They're not really email addresses in the sense you think -- they bounce (or sometimes redirect, but mostly bounce). They're just Twitter-identifiers.
(This is a made up example)
Does Goodreads provide email addresses?
Are these potential friends goodreads officials?
What is the tie up between twitter and goodreads?
Thanks for satisfying my curiosity.
Rowena