Do I need to make a reservation or can I drive there Friday and get a spot?

Reserve Ahead.

If you're planning on staying the weekend it's best to make a reservation otherwise you will be moving from one campsite to another. At San Clemente reservations are site specific, people choose beforehand what campsite they are going to stay at. So if they do have a campsite open one day it might not be the following day and you will have to move to another campsite if one is open.

Reserve it

They book up fast but you can try

We booked our spots 6 months ahead of time, but there were a bunch of open spaces while we were there. So maybe?