Depends what you bring to the table... Our camp *SOMETIMES* accepts members post-opening, but they have to put up with a lot, and bring a lot to the theme, camp, and interactivity, plus pay all applicable camp dues plus a penalty tax.
Trust me, it's not something we normally do... and we're pretty easy going compared to most.
People who show up and flake usually find their camp gear neatly strewn about the road in front of the camp, thence to be processed into disposable chunks by the parade of art cars that float on by. (Actually, they are asked to relocate.) We get stellar placement and get asked by a lot of last minuters who have nothing, but *swear* how they'll do all this stuff for the camp. Guess what happens about Wednesday? Guess how much our moop quotient goes up? Guess who gets stuck cleaning up soeone elses mess?
Besides... why do you need a camp? You can camp anywhere on playa pretty much, and not put up with assholes (like me)!!