Launch a contest on your website.
Give away something to the winner of the contest as a motivation to join. This has been a proven method to get new users to try your website.
To get the world out, find a popular blog related to the topic and offer them exclusivity on the contest information.
Speek, one of the best conference service out there, outdid everyone when they hosted an event where you could enter into a contest to speak to Edward Norton, the famous hunk who starred in Fight Club and American History X. Here's the event page: http://blog.speek.com/site/win-a-speek-conference-call-with-edward-norton/