You have to consider what benefits you have to offer contributors, and market those in a concise way. Do you have a blog? If so, you could run interviews and guest posts which offer value to your users and advertising for your contributors. Can contributors work with your startups in a paid way, similar to Clarity? In which case, what makes your site more useful or better than Clarity? Have you designed your business plan with both B2C and B2B considerations in mind? This may take doing a second business plan or finessing your idea.