From The Vault
Upon entering the main building of the Khan camp, talk to Garl, the leader of the Khans. Tell him you want to join, after this say that you want to kick some serious ass (choosing any other dialogue option will result in the Khans attacking you). He'll then tell you that he tires of his girl slaves, and that you must prove yourself by killing them. Doing so is easy as they cannot fight back and will simply run away, but be sure to get them before they leave the camp or the Khans will turn on you. Once you have done this Garl will name you a 'friend of the Khans'. You will then have free rein of the camp.
