You should have a proof metric - market traction, market proof, proof that they are not wasting their time on your site with half-baked ideas. You should also have some way to show that there has investigation and transparency into the crowdfunders. Try a group with vetting services like http://clearbusinessdirectory.com/vetting-services/ Lastly, you should have clear focused categories that are not overrun with misfits. Investors may want to focus only on a particular area of interest. Make that clear.
I think there's a huge educational gap for retail investors and the crowdfunding platforms are positioned well to resolve it. You can market your platform the way you would any other web site - define the target group you want to bring to your platform, and then use proven online marketing strategies to reach them. That could include Facebook ads, webinars, guest posting, partnerships, SEO and other tactics.
When these people reach your site, educate them about how to invest in the equity crowdfunding model. Don't make them believe equity crowdfunding is a get rick quick model - show them how patience and brand alliance can help small businesses succeed and celebrate the role they will play in that.
(1) Provide a detailed business plan which shows the route to return on investment for investors.
(2) Make sure you promote the platform where your potential investors are most likely to come across it.
(3) Make sure you have prepared detailed answers in advance to potential investors questions.
Those are 3 useful variables to consider in creating the best go-to market strategy for acquiring investors on an equity crowdfunding site