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The thing is, people aren't going to wait. They come to the site, see that if they want to join they'll have to do it in three weeks, and promptly return to whatever forum they came from. Prospective members aren't invested in the site enough to wait. I suspect the result of such a restriction would be only people who found the site on a sign-in day would join, lowering the number of recruits but not having much of an impact on their quality.