When I have invites, and dribbble usually gives out 2 at a time.. I actually ask designers to email me their work... and yes I actually review all 70-80 portfolios that come in and pick 2. It isn't easy work but hey I know people really want to be on dribbble. I do find it mind blowing how people will send 1 attachment of a portfolio item. I also find it mind blowing how rude some people are in the way they ask for an invite and / or beg for it.

Keep sending out your portfolio and hopefully you will get picked. I've seriously looked through about 80 portfolios last time and had to pick 2... it is super hard to pick just 2.

Pretty much what Dribbble has become. Not many people actually give feedback