Hi Xianghua,
We've already flagged the display of available variables as a feature request: see
http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=4564 (and please feel free to CC yourself to that report for future updates).
Unfortunately, I don't believe there is currently a variable available that adds a user's email address. There are a number of threads in the forum that describe how to add variables to the system to be used with prepared emails; you should be able to find them by searching for words such as "prepared email variable template", etc. If you have any specific questions, let us know!
Cheers,
James