Check this link : http://www.secret-japan.com/forum/harajuku--amp-omotesando-restaurants-american-food-t189.html
At this address, you can't be wrong :
Barbacoa Grill - Brazilian Churrasco ****
Address - Tokyo-to, Shibuya-ku, Jingumae 4-3-24, Evergreen Bldg B1
Opening hours - 11:30 to 15:00 and 17:30 to 23:00. Open all week.
Telephone - 03-3796-0571
Menu - In Japanese and English
Credit Cards - OK
It is not really a steak house, but rather a meat restaurant. Still, the meat is the best
I had so far in Tokyo...
Plus, for a party, it is really pleasant!