Only the people listed on the lease are to live in the apartment. Anything else is a violation o the lease.
Most municipalities have regulations on the number of people allowed to reside in a dwelling, by size, number of bedrooms and by the age of the occupants.
You should be able to call 211 to get that information for your area.
Let's say at present you and a friend are sharing the apartment. Your friend wants her boyfriend to move in. In order to legally do so he has to be put on the lease, not necessarily as a lessee but as an authorized occupant. It would be the same if she had a baby. There is a limit to how many are allowed to live in the space as well.
at 6:05 PM on Oct. 21, 2013