It was easy to attract members a decade ago but now when the competition is just brutal, that's everything but an easy task. Visit your website and ask yourself - "Would I join this site?". Does your website offer something new and unique? How is it different from hundreds of other cloned websites?
You must offer something fresh and new to people, don't try to sell them your copypasta. If your website is one of those Yet-Another-Music-Website then it stands a pretty good chance of failing.
Anyway, if I were you I would first try to "bribe" some of my targeted traffic. Offer 5$ - 10$ for a free sign-up to first few hundred people and then just make a good marketing campaign and voila.
To find your victims, go to http://www.soundclick.com and start advertising. Many of them already sell music through Soundclick but since we're all money-hungry I don't think they'll say NO to you.
I really can't believe you haven't thought of Soundclick in the first place, it's old as a Bible and the biggest website of its kind on the web. If you won't be able to find music producers interested in your website there then I don't think you'll find them anywhere.