go check out the pacific northwest region pca website. It has all the answers you need.
In short, this event is open to pretty much anyone. Being a BMW CCA member is probably the best bang for the buck.
Sign up for a motorsportreg.com account. Go sign up for this event. It is that simple.
If you need a club membership I recommend the BMW CCA. they have a bunch of events during the year, they have a good relationship with the PCA, which gets you into PCA events as well.
The ELCC is also a good group and seems to be building the relationships in the area. ELCC is the Evergreen Lotus Car Club.
Now get those cars out to the track! We need more of the northwest cars out at the track!