Ok I'm purposely not going to get into some of the stuff that I would like to address thats been said in here, because well, basically it's not constructive.
What it comes down to is, who does it hurt if the site keeps allowing new people to join and participate?
It's a brand new site, it's only been even publically known about for something like 2 weeks. Maybe if this keeps going in 2013, when people have had a whole year to discover Weekly Beats exists it will get closed down to new participants a week in til January.
Maybe it will happen in a few months if the site really takes off. For now allowing people to take part later in the piece is IMHO only going to make it a tighter community.... Seems to me people like to introduce their friends to their online viewing (or participating) habits. It seems to make them feel more like they're part of something.