HELP GETTING HALO 3 MAPS ON XBOX 360

Hello, does anyone know how to download Halo 3 file shares on Xbox 360 today?
There was a way using the chat invitation, but the option to enter a new gamertag to contact is greyed out for me.
I don’t want to use any pc programs, just the Xbox 360 interface.
Arise all you fanatics and techno-experts!