If you didn't know already, Dribbble is an invite-only network and invites are pretty rare. You only get the chance to invite someone very rarely and they come in very low quantities. This is done so that only the best designers can join and show off their work - it keeps the quality of the site very high.
I have just received two invites to giveaway. To get your chance at getting into the network, just submit a link to your portfolio in the comments and follow me on Dribbble. Make sure to have a way to contact you on your portfolio so I can get in touch if you are chosen.