Re Tigerman's recommendations:
Owen & Engine - fantastic, was there last Saturday. They have a private room upstairs that should be perfect for twelve.
Urban Belly - great, but do twelve hungry Scotsmen want to sit in a stripmall parking lot waiting for a table?
Slurping Turtle - haven't been
Au Cheval - same
Frontera Grill/Xoco/Topolobompo - Frontera does not take reservations, Topolobampo does - and it is a can't miss when in Chicago
DMK Burger Bar - good
Fish Bar - haven't benn
Arami - same
Macku Sishi - same
Girl & the Goat - really good, Michelin starred I think. Make reservations now - hard to get a table.
Arun's - do not go to Arun's. Overpriced and really not special anymore. I live a half mile away and I never go there.
The Bristol - real good.
GT Fish & Oyster - never been
Lao Shanghai - never been
Mexique - I love this place.
Smoque BBQ - Fantastic. Almost as good bbq as mine. No reservation need but bring your own beer and wine.
Spacca Napoli - not my favorite. Coalfire is better.
Twin Anchors - eh
Pizzeria Due - I have not been here since high school.
The very best restaurants in the city (I haven't been to Alinia, Next) are Schwa and Goosefoot. Don't bother going there with twelve guys after golfing. In fact you couldn't get at table at Schwa for sure (biggest table is six). Best thick crust pizza is in Burt's in Morton Grove - but ordering there is also a pain in the ass since you have to order the thing a few days ahead of time. But on a northshore day it is on the way back to the city it is great.
Sepia: this is perfect. I have been going here for years and have never had a bad meal. I kinda know the chef and he is immensely talented. My favorite steak is from Sepia. There also is a private room in the back where I'm pretty sure 12 can sit comfortably.
When in Chicago you should also try at least one high end Mexican restaurant. Topolobompo as stated above is the best bet. But if you want to get into the neighborhoods Mixteco is real nice and Sol de Mexico has some famous moles.
On the next Check, Please...
on edit: I belong to two message boards. GCA and lthforum.com - a local food message board. Go there, search around. This link (I can't figure out how to put links here )
http://www.lthforum.com/bb/viewtopic.php?f=28&t=32598 are their Great Neighborhood Restaurants. These tend to be more ethnic, cheaper but really good spots.