Perfect Brunch Spot for 20 in Seattle
You might think about dim sum options if people are in to that type of thing. Most of the dim sum places around town could easily handle that amount of people.
Otherwise, I wouldn't say that Seattle is too much of a brunchy place. You might consider some of the breakfast places that are recommended in other postings.
Julia's on Queen Anne is great, they have a large room upstairs. They don't take reservations, so somebody will have to go 30 min. early (bring a book). The food is wonderful and cheap(ish), and you can choose to sit in a long row, or in small tables. Since it isn't a buffet, you may want to order in small groups, but the ambience and food is great.
Gosh, for twenty people I'd try someplace with a buffet. That way everyone gets what they want, the wait for food isn't long and no one feels bad while a loved one has to eat their over easys cooked hard. Buffets also seem (imho) more fun for a large party because you can easily mingle instead of being stuck in your seat.