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.

WHERE
There is tons of communities you can tap into, here is a few:
http://www.slacklist.info/
https://www.quora.com/
https://www.producthunt.com/
https://news.ycombinator.com/

https://www.reddit.com/r/business
https://www.reddit.com/r/inventions
https://www.reddit.com/r/marketing
https://www.reddit.com/r/motivation
https://www.reddit.com/r/sales
https://www.reddit.com/r/smallbusiness
https://www.reddit.com/r/socialmedia
https://www.reddit.com/r/startups
https://www.reddit.com/r/Consulting
https://www.reddit.com/r/BusinessHub
https://www.reddit.com/r/contentstrategist
https://www.reddit.com/r/digitalnomad
https://www.reddit.com/r/EntrepreneurRideAlong
https://www.reddit.com/r/growmybusiness
https://www.reddit.com/r/hwstartups
https://www.reddit.com/r/mutualcollaboration
https://www.reddit.com/r/socialpreneur
https://www.reddit.com/r/SideProject
https://www.reddit.com/r/entrepreneur
https://www.reddit.com/r/kickstarter
https://www.reddit.com/r/AlphaAndBetaUsers

HOW
Join the community
Contribute to the community
Make a post where you:
Outline what you need
Outline what you offer in return
Provide clear and easy Call to Action

Contact the opinion leaders in your area or startups with a strong brand who blog about the topics you want to focus upon. As previous poster pointed out, find communities with high-level of contributors that are actively engaged. Crosspolinate the best posts across other social media to present your community as a knowledge leader and leverage key words that resonate with your audience.

As per previous post, for active users there has to be something in it for them. Sometimes it is the quality of the discussion. Sometimes it is to build their existing brand as a knowledge leader. Perhaps you have perks through partnerships with other services. These are starting points. Good to constantly survey your existing base to understand what is working and what is not working with your existing service.

LinkedIn is a great place if you are looking for contributors set up a business page for your website on LinkedIn and link it to your LinkedIn personal page. Then post several blog articles or recorded interviews to your LinkedIn blog with a link back to your website. Include a "call for contributors" link so readers and others can share it or contact you.

I have used chat feature in my website to keep them engaged. The chat is anonymous and starts without needing a login. This have made my visitors spend more time on the site. The best community to target talented people is Quora. I have got so many visitors through Quora. You may like to try that.

First question you should ask yourself:

What value am I bringing contributors?
If you are going to pay them (much like this site) for their time it's an easy sell and you should make a hit list of the most asked questions (80/20 rule) 80% of revenue will come from 20% of your requests.

Then make a list of experts in those fields you find through researching the web and then offer them % of the revenue to answer or help with the questions.

If you're asking for where they are the answer is on all social networks (if their worth their weight in GOLD).

If you have customers asking a lot about a certain service
i.e.
mobile interface design

It should be really easy to make a list of mobile interface devs who are willing to get paid by you to help others.

If you're trying to sell leads to them, give them a few for free and get feedback from them.

Did they get business from it?
Did these people looking for services flake?

RESEARCH will get you everything you want. It's a lot of work but if it were easy, everyone would be rich with business.