I think it’s pretty easy to create a TweetUp.

Make sure you have enough people from your area who are on Twitter and who are connected and talking to each other.

Pick a date (take a poll if you’re unsure when), and a time (after business hours probably good), and a location that is central, popular, and has room for your group.

Then, annouce it! Get all your people to RT it. Use http://www.twtvite.com to take RSVP’s.

We had our first TweetUp in Fresno, California last week and it was amazing. You can read about it here: http://is.gd/kjFn

You’ll have no problem in Seattle. I think your problem will be finding a big enough space

Good luck to you!