Harriet and Brandon do not have public email addresses. However, all is not lost!
You can write to Harriet in care of Tor Books. They are happy to forward mail to her.
c/o Tor Books
175 Fifth Ave.
New York, NY 10010
Brandon maintains a blog at http://www.brandonsanderson.com/. There’s a form you can use to email him, as well as a link to his official forum. He is also on Facebook (Brandon Sanderson), Twitter (BrandonSandrson) and Livejournal (mistborn). Keep in mind that Brandon is very busy writing and may not have time to respond to you personally. He uses these mainly to keep fans appraised on his progress with his various projects as well as his appearance schedule.