Beyonce just hears those harmonies in her head. She was born with them. Her dad, Matthew is quite a good singer, and I believe Tina sings also. She usually starts off with the tonic, and builds from there. She does a little experimentation, and then blends them herself with the engineer. I love her timing. She has no regard for the beat, but it always sounds right. I don't think I have ever seen anyone work harder than Beyonce (maybe Mya). I've seen her fly in from Europe, come to the studio, do vocals for 4 hours, fly to NY for concert, go directly to airport for 2 shows in Japan, and on and on. No sleep for days, and just keep going and loving every minute of it. Mya is like that also. I get tired just TYPING about it!
http://www.gearslutz.com/board/q-dave-pensado/21012-lets-talk-about-vocal-stacks-especially-brian-mcknight.html