In my state all responsible parties must sign. If the lease is for jan 2006 - Jan 2007 then another lease is drawn up & again everyone signs.
If a lease runs out and the landloard does not draw another lease but allows the people to continue to rent, The landloard must go to court and show proof that NAMED people lived there and owe $$. The landloard can NOT use an old lease as proof. They would use name on utility bills and go after that person.