In most leases, you are required to notify your landlord of any occupant over the age of 18 who stays in your apartment for more than 8-ish days (I say “-ish” because every state is different). That doesn’t mean tenants do, but the rule is in the lease for these, and other similar situations. For example: one person (leaseholder) rents a unit (likely because the “occupant” wouldn’t qualify due to their credit, income, criminal background, etc.) and they (the leaseholder) either never move in, or are only in the unit sporadically and the occupant, (not technically a leaseholder and therefore not technically bound by the terms of the lease) is the main resident. Based only on my personal experience, these situations are usually a mess, and almost always end in damage (not only to the unit they occupy but others as well), violence or threats of violence, and an unbelievable waste of resources for a landlord/property management teams (who aren’t the ‘bad guy’ landlord). Are there times when it works out, sure. But more often than not, it ends in the courts, sometimes for years, which is an additional waste of resources.