Granting Access to certain files based on create date.

Hey Guys,
I am need to figure out a way to grant access to a website based on a members create date. An example would be a member joins and on that day they have access to the first level of our program 4 weeks later they get access to level 2 and so on. I don't even no where to begin I can't find any documentation on the subject so if you guys could turn me on to some it would be greatly appreciated.
 
Thanks  
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