A roundup is an article where there are a number of contributors. So here you want to think about an article topic and then find a number of people on Facebook to contribute to it. You need to choose a topic that will be popular with your target audience.
After writing your article, post it on your website. Then create a Facebook post telling people all about the article. Ask the people that contributed to the article to share your Facebook post with their friends. This is a great way to get more free traffic from Facebook.
Use a List Post with credits
Roundup posts are really good but they take a great deal of time to create. As an alternative you can create a list post which mentions and provides credit to others without them having to do anything.
So for example, if you are in the Internet marketing niche you could write a list post like “10 reasons why you must do email marketing” and then mention people that you know in the industry and credit them by using their words and quotes etc.
In your Facebook post you can tag these people as well. Also contact them directly using Facebook messenger and send them an email as well telling them about your post and the fact that they are mentioned in it.