There's no equivalence of 'you' in Korean. We call younger friends by their name and call older ones 언니/오빠 or 누나/형. So we say like this, "언니, 바빠요? Are you busy?". If the friend is younger than you or your age, you can use 너.

Yea, it's not really correct to use 당신 in Korean, you hear it very little in day-to-day speaking. In Korean the word "you" isn't at all like we use it in English. Most of the time, Korean is spoken in a way that if the subject of a sentence is "you" then it will be very clear from context. So no need to even use the word. But in the situations where its necessary to use, or the speaker just feels it is right. . . then they will say the other persons name in place of "you". "너" and "네 [니]" is also used to replace "you" in some contexts..