Terminal 5 is the check-in area for US carriers. Once through security you'll transfer via bus to the main terminal area.
Once in Satellite "G", you should have access to Alitalia's Freccia Alata - Giotto lounge in the center of the terminal.
Nice lounge, was there last month.
If you want to view photos go to:
http://roma.repubblica.it/multimedia/home/21977422/1/1