I've used the walk-in campsite at the Portal 3 times. I haven't had a problem finding a spot to pitch my tent, but I can see where it could get filled up on a July/August weekend.
Reservations are not allowed and it's first come/ first served. There is no campground host. You will need to find an empty 'pad' to set up your tent and then fill in the self-registration. Last year the fee was $10/night. It's not the most quite of locations, but it is right across from the TH.
I'll have to search for the post that had the plans for additional walk-in sites at the overflow lot. If I recall correctly, they were planning on having them open by July.
And I'll second Steve's recommendation of Horseshoe meadow. There are 3 different campgrounds, all walk-in, no reservations. And 10k elevation.
