I think all of the camping at the portal is first come first serve. I am driving up tomorrow to day hike on Sunday. I will let you know my experience, including finding a campsite on a Friday as you will be, when i get back.
An alluded to earlier, 60% of the campsites in the family campground are reservable on Recreation.gov. The rest are FCFS. The walk-in backpacker sites by the trailhead are also FCFS. (See also Whitney Portal Campground)
I got there around 4pm on Friday 7/20. The group campsites were full but I spoke to the lady in charge of them (Cheryl?) and she told me to come by her camper at 530am Saturday and there would most likely be a spot. I was lucky enough to get the last backpacker site up by the trail head. I woke up at 5 walked down and got a spot no problem.