Landlords, if they're business savvy, take great care in approving and declining applicants who wish to be tenants. This "screening" process is meant first and foremost to weed out those who present a sizable financial or safety risk. In other words: deadbeat and criminal tenants. Property owners and managers take a gamble that whomever they approve will be responsible with paying the rent, not tearing the place apart and not posing a nuisance or menace to neighbors.
They want to know -- reasonably enough -- who's living in and on their property. Thus, expect some kind of restriction on the length of time guests are allowed to stay. Around two weeks is a typical amount of time a lease will permit a non-tenant to reside in the unit. Any longer than that, and the person will likely have to have his or her name added to the lease -- perhaps with a higher monthly rent, perhaps not.
It's good to know ahead of time what the policy is, so that you don't get a nasty letter when your couch-crashing pal from out of state garners the property manager's unwelcome attention.
When the lease is first drawn up, come clean with the landlord about who will be living in the unit -- be sure to include the number of people (including children) and their names. Do note that the law prohibits landlords from discriminating against potential tenants just because they have children (except in special cases such as retirement communities).