Don Vito says "Bienvenuto al la Casa Nostra" to Barzini, When Barzini arrives at Connies wedding.

It's not in the trasncript, and when he says it in the movie you have to listen carefully.

How it goes in the movie.

Don Barzini arrives with his men
VITO: Eh, Don Barzini.
They hug each other
VITO: Bienvenuto al la Casa Nostra.

How it goes in the trasncript.

Don Barzini arrives
VITO: Eh, Don Barzini.
then, Vito introduces Barzini to someone, in Italian

I doubt someone has the name casa or cosa nostra.

In the beginning it was never about good versus evil, it was all about power.