You could advertise in the right places to get members. Many web site owners would jump at the chance to sell a banner space to you for a few tens of dollars. The best way to do this is to approach them and say you’d like to put a banner on their site for X months for $Y. Make X twice as long as you need and $Y half as much as you will spend. Built in negotiating room. You could also offer the web site owners a % of the profits. This of course reduces your risk, but could reduce your profits. Other ways to advertise is using Google Adwords - use banner ads and text ads and target very specific niches. Facebook advertising is ideal for targeting - you can target people with interests and fans of pages etc. Massively powerful.