How do you reserve a space

All the times I've gone, I've just driven in and paid, no reservations were required. If you have a large event where you need to reserve tables, BBQ grills, you can contact Marin County Parks and Open Space Dept. 3501 Civic Center Dr. San Rafael. They should be able to make reservation's for you.

Reservations at the nearby campgrounds are made via the state park reservation system: https://www.reservecalifornia.com

It's a state park. Maybe through the state park system. Not all the spots can be reserved. Many first come first serve spots. Visit it first.