One way to promote is to buy links, if you want we can arrange that a blog post with a review of your site be posted on a large number of blogs, 300 blogs some with google PR but all the blogs are indexed. your site would be on google in a few days and depending on the keywords you are betting they might even come on top. (you write the review) . Anyway if you are interested on this we can arrange to promote your site for a few months. (the price depends on how many links you want a month. it is not very expensive actually, 100 links-$30 a month, 200 links -$50 a month, 300 links $75 a month. we dont do more then that for new sites because google for new sites if they get too many links in the first months google may suspect and penalize the site.)
Actually what are the keywords? what do you think would be the best
description and meta keywords for your site? if you decide to give us the task of promoting your site we would also optimize the template, (alt tags, etc.)
I think also that you should not charge for membership, at least at this stage, one way to promote it would be to offer free membership to the first 500 members.
Also any plan you choose we will study the keywords you want and choose the best keywords based on the possibility to rank well on search engines to promote the site.
Last, you must understand that your site will not rank well for some months some people say 6 months, so you must be prepared not to have any income for the first months.