Most of the time — but not always — the people listed as exemptions on your federal income tax return (or your parent's, if you are a dependent student) qualify as members of the household.
For dependent students:
The student and the parents with whom they reside always count as members of the household. Brothers and sisters of the student also count, if the parents provide more than 50% of their support (though they may not necessarily live with your parents). Other people count as part of the household if they lived with the student's parents last year, will continue to do so from July 1 this year to June 30 next year, and will receive more than 50% of their support from the student's parents.
For independent students:
The student and their spouse (as long as they are not separated or divorced) always count as members of the household. Other people count as part of the student's household if they lived with the student last year, will continue to do so from July 1 this year to June 30 next year, and will receive more than 50% of their support from the student and/or the student's spouse.