PDX Place for Group
You should try Henry's 12th Street Tavern in the Pearl! They have a large restaurant area where everyone (21 and under) could dine in. I'm in my early twenties, and while I know there are places that serve better food, I think they'd enjoy being at Henry's with the fun lounges, big screen tvs, billiards, and trendy setting. As long as you stay within regional menu selections and away from their interpretation of Chinese food, you'll be fine.
Malay Satay Hut has a huge menu, very good food and lots of room (in the Fubonn Shopping Center):
For a more upscale experience, you can get the private room at Tabla where you can order off menu (they have a great 3-course for $24 deal) or arrange a family-style meal: