Hey there, Jedi.
According the fan sites, there is some randomness involved so it's not going to be all day 1 people, day 2, etc.
My guess is they are bringing young wizards in based on age, country, languge, etc. which makes sense for beta testing. They did have some demographic questions during registration which enables them to get a decent cross-section.
Guess we can only wait. Early reports from those who have gained access indicate that the graphics are beautiful, Rowling's additional material is outstanding, and the site was well worth waiting for.