Probably the two most common sites for Chinese banquets are Empress Pavillion, 988 N. Hill St. in Chinatown and NBC Seafood, 404A S. Atlantic, Monterey Park. Other good choices with the ability to host a large gathering include Ocean Star Seafood, 145 N. Atlantic, Monterey Park; Empress Harbor, 111 N. Atlantic, Monterey Park; Ocean Seafood, 747 N. Broadway, Chinatown; Sam Woo Seafood, 140 W. Valley, #D, San Gabriel; 888 Seafood, 8450 E. Valley Bl., Rosemead, and others too numerous to mention.
Some non-asians find the ingredients and styles of Southern chinese food strange and disagreeable. If that's the case, and you're dealing with Europeans especially, I'd look at Quanjude Beijing Roast Duck (in Rosemead) restaurant. They have a banquet room and set menus, and tell them that you want mostly NOrthern dishes.
Also try Tung Lai Shun in the Great Mall in San Gabriel which can accomodate banquets or the Chinese Islamic Restaurant on Garvey near Del Mar in Monterey Park which is less ornate but can accomodate large groups. BOth restaurants serve a type of bread with scallions (many non-asians have a problem with a meal without bread) and specialize in lamb dishes. Good luck.