Try to find out what other platforms online your target audience already uses, and then learn their habbits, might also want to use advocates to recommend your platform ( but very gently and in a smart way). You most likely need to compensate those advocates. Consider finding out ways to get the emails of  your prospect audience ( not easy, need to be compliant) and invite them via email. Use multichannel approach- open accounts on popular social networks and promote your site by providing valuable and relevant content.