All visitors for business or pleasure are principal applicants, even if you are traveling with your family. If you are coming to the United States in a category of visa that permits you to work, study, or get married, you are the principal applicant. You are not the principal applicant if you are accompanying your spouse or parent who is coming to the United States in a category of visa that permits them to work, study, or get married.
看来,若是以旅游探亲为目的,每个人都必须是主申请人