Oh this is too easy.

A. Create an advertisement article writing contest awarding 100 steem to the article that best promotes your project. It should also probably demystify your project for example I still don't know how winners are chosen exactly and well, I'm having troubles finding that answer on your site.

B. Let the contest run for a week, resteeming the contest article should be a requirement. Make sure to announce there will be a writing contest 2 weeks prior to the announcement of the contest. That way you can plant the seeds in people's minds and they can get a headstart on writing the article before the contest is even officially announced.

C. Pay good content creators in either liquid steem or votes if they wear a banner ad that links to the winning advertisement article that best sells your site. All you really need to do is introduce people to the concept to sell them on the benefits of using your site.

D. Make a video or employ someone to make a promotional video the shortly and consicely explains how simple steem-bounty.com is to use. You really could spend as much or as litte as you want on a project like this and still be successful. I think the idea kind of sells itself. It's just a matter of packaging the message.