Where do you see the most engagement?
Think about where you tend to find the most engagement - social media (likes, comments), company intranet, your website, blogs, forums. You will want to promote your project somewhere that people are likely see and interact with your post.
Pick those who are right for the story
It's worth considering the people who are best placed to comment on the topic that you're sourcing video for. If something is important to people, they're more likely to join and contribute powerful stories.
Speak to your marketing team
They hold the data on where you can get the most engagement and what the best time to post is. If you don't have one, use the analytics pages for your social pages and websites to retrieve some useful data on who you could target.
Involve your known champions
Do you already know the people who are most likely to take part? Reach out to them directly. They can help get the ball rolling and in engaging other people to get involved.
Reach out to team leaders
Are you trying to get employees into your project? Can team managers help nominate or reach out to their most suitable team members?
What's the incentive? Think about what you can offer or how you can frame the project to show the value in people taking part.