Unfortunately, landlords do not have control as to who is living in the unit, and the tenant is not obliged to provide any information about their roommates or guests either.
You are not required to draft up a new rental agreement as the original tenancy continues with the remaining tenant. The tenant is not obligated to sign a new rental agreement either.
As for increasing the rent, you cannot increase the rent based on new people residing in the unit. You can only increase the rent legally by providing a proper 90 day notice on the prescribed form (N1) every twelve months for sitting tenants and following the guideline amount.