Back Bay for a Group of Clients
IF you want other steak houses: Grill 23(power scene,try and get a private room) and Capital Grill (I prefer the other 2 over this one). Or, you could go to Clio's, which has excellent food, great atmosphere, sexy yet...sophisticated. Or what about the Bristol Lounge in the Four Seasons, not as upscale as their formal dining room, but still great for entertaining clients.
I don't know how casual you want to go, but the Cottonwood is definitely more on that side but can serve decent southwestern food. You might see a celeb at Ciao Bella, they always seem to go there but again, I don't really see it as a client destination. Never been to Casa Romero.
Vox Populi has a pleasant dining room, a couple doors from Abe & Louie's. Lots of choices of upscale American fare. As someone mentioned, the Bristol is very nice; getting out of Back Bay almost at that point. Sonsie has a private room if you have a large party. And there's always the Palms in Copley Place and the Oak Room at Copley Plaza if you want meat.