Good Little Tokyo restaurants for group of about 15-20?
I work in the area so I know it pretty well.
Suhoju is a newish westernized Korean restaurant in the Little Tokyo mall and it could accommodate that many. I've been there twice, including today for lunch. It's gotten mostly favorable reviews on these boards, you can do a search for it. I liked it both times and my friend liked it a lot as well. Chap jae, bimibap, nice veggies, grean tea rice... it's all good. Very nice modern decor/ambience too. Reasonably priced. Very friendly staff. No kim chi though, which some have been horrified by. I don't miss it.
After you can all go to "If" fka "Fiore" and try their Pinkberry knock-off yogurt.
I've also been with a large group to Wyland's Brewery. I think their food is just okay, but it's pretty mainstream which sometimes is the only way to go in a large and diverse group of people.
Senor Fish could also accommodate that many, but again, I think their food is just okay.
Sushi Gen in the Honda Plaza is my favorite place to eat around there but it would be very hard to get that many people in for lunch. If it's for dinner, I don't know, I've never been there for dinner. I just know it's insanely crowded at lunch -- it's an ordeal even when I go alone. Also, don't know if pricey sushi is the way to go with a group (only the lunch specials are affordable.) Not everyone likes sushi, not everyone likes to pay a lot for sushi.
The sushi and tempura bars on roof of the New Otani are indeed very nice, but they're pricey too.
If you don't like bad food, don't do the New Otani buffet downstairs. If you and your group do like bad food, the buffet is perfect.
I understand Kanimura is only opened for dinner. Never been there but heard it's good.
2 places that might be able to accomodate your group might be: 1) Yatsuhashi on San Pedro downstairs in the basement below California Bank & Trust. They have teppanyaki (Benihana style cooked in front of you) I've gone to parties there before and I believe each grill can seat about 8 or 10 people. I believe there are four grills arranged within view of each other.
2) Kushi Shabu on the 3rd floor of Weller Court. They have an all you can eat Shabu Shabu and a private tatami room that can hold that many people easily. The tatami room has an open space underneath the table for your legs so you're not actually sitting on the floor.
Either place I would recommend making reservations in advance.
You could probably do something at Sushi Gen if you arranged with the staff beforehand.
I've also been with a big group at Kani Mura, a crab restaurant in the same mini-mall. Very fun, but you gotta love crab.
You could also do Koshiji, a yakitori place next to the Curry House & Kinokuniya Bookstore. One of my favorites in town.