I have two close friends. What happens to the other three people? Do I get to pick them?

...Only if my girlfriend/future wife isn't included would I ever consider this.

Its your five closest friends. If that includes your spouse/family, then it happens to them. If you only have 2 close friends, its your 3 closest acquaintances. You don't get to pick. Some cosmic force that just "knows" decides.

1/3 of 20, 6 people die and one person losses 3 limbs?

I don't have ANY close friends, let alone five. let the billion come! Then, off to Moscow to fund the WORKERS REVOLUTION!!!