Legal Question in Landlord & Tenant Law in Illinois

If the lease specifically does not mention the additional occupants (some forms allow you to put in the additional occupants by name or relationship), or by its terms does not include them automatically (for instance "immediate family", etc.), your remedy is an eviction against the "illegal" occupants, preceded by an appropriate landlord notice. As your posting discloses an Evanston address, if the building is in Evanston or Chicago in particular you may be required to follow the applicable landlord/tenant ordinance which could create additional requirements. Best bet would be to have an attorney review the lease to make sure it in fact doesn't cover the additional occupants and/or the applicable ordinance doesn't grant them some rights, before starting WWIII.