As was mentioned, in the above 2 posts, PSAS is a great club, in the Puget Sound area!
There's also Rogue Reefers, which is an informal group of hobbyists, most in the South Sound area. Information on the Rogue Reefers can be found here,
http://www.reeffrontiers.com/forums/f230/hello-everyone-id-like-introduce-rogue-reefers-59046/
SeaMax, held in Seattle, July 23-24 will be a great way to meet local hobbyists, in your area, as well. I'll be there, as a PSAS member and as a Spokane Reef Society member, since I'm a member of both clubs. Information on SeaMax can be found, here,
http://www.marineaquariumexpo.com/SEAMAX INDEX.html