Backpacking spots for a big group

i’m leading a group of 10 boy scouts out near portland. Where should we go to get a wonderful experience, but find campsites to accomodate all of us?

One suggestion is the Cascade Locks Marine Park, probably 30 minutes away from Portland in the Gorge. It has a safe, friendly, open and kid-friendly vibe, lots to explore for the scouts, several gorgeous river access views, and includes historical aspects and the nearby Pacific Coast Trail.