How do I enter bosses only on Diablo 3?

[Sambaria]

So I've cleared Diablo 3 on normal difficulty and now I wanna clear some bosses with a friend, can I teleport to the bosses straight away or do I have to clear some dungeons before I'm able to kill the boss?

[Delaios]

The only way I was able to re-do bosses, is by leaving the game, changing my current quest until the closest point to the boss and re-clear from there. After logging in you take whichever waypoint is closest and re-do the quest as usual.

It's a bit more cumbersome then it was in Diablo 2.