Full fenders with a splash guard are a must for group rides here 9 months out of the year although you can often avoid the rain with a flexible schedule and watching the local Doppler radar.
I've never been a member of Cascade but as mentioned they do a lot of weekly rides of all levels. I've been a member of the Mountaineers for climbing but they have a biking group. Membership is decidedly on the older side to the point that they are trying to grow membership. Both have websites you can peruse.
There are a lot of biking clubs in the area.
Bike Clubs in Washington StateWashington Bikes