The rule against subleasing extends to situations where friends or family move in with the renter. Having a temporary visitor at a rented apartment is normally not a cause for legal action on the part of the landlord, but a problem arises if the visitor begins living at the apartment. If someone other than the renter begins living at the apartment, the additional party needs to also be on the rental lease.
A third party who lives in the rented apartment but is not on the rental contract could produce legal issues for the landlord and many landlords won’t allow this practice. Renters should be careful when asking someone else to live with them.