Who are the "important" people in Kabul? Why are they called "guests"?
Farid calls Amir a "guest" in Kabul because he is a Pashtun; a member of the privileged, or "important", class in Afghanistan. Pashtuns represent 42% of the population and dominate governmental and legislative bodies. While there isn't a rigid caste system in Afghanistan, the diversity of ethnic cultures does create distinct socio-economical situations.
As we see in The Kite Runner, Hazaras are viewed as being at the bottom of the ethnic hierarchy. Hazaras typically hold service level jobs and make a menial living.
Amir is a guest in Kabul because as Baba's son, a well-respected and successful Pashtun, he has lived a very sheltered and protected life compared to other citizens of different ethnic groups. Amir grew up in a big house guarded by gates which blocked out the "real" experience of Kabul.
Amnesty international has a great guide for The Kite Runner with a very comprehensive breakdown of the ethnic hierarchy in Afghanistan. I sincerely suggest you check it out.