When you sign a joint lease, you and your housemates are all taking responsibility for the entire lease, not just a portion of it. This means, one person will be designated to pay the full monthly rent amount to Portsmouth. It is up to you and your roommates to pay each other your own portion of the rent.
When you sign an individual lease (by-the-bed lease) within a shared house, you are only responsible for what is on your lease, you are not responsible for your roommates.