A lease is a contract; while it is in force, neither party may change its terms or conditions. (Or more accurately: neither party may change its terms or conditions without the consent or agreement of the other, so in theory you could agree to pay more, if the landlord somehow convinced you it was fair or appropriate to do so.) Therefore, unless the lease itself has some clause or paragraph which would allow rent to be increased if the number of people in the household increase, the landlord may not raise the rent. Of course, when the lease is up, the landlord may seek an increase for any renewal.