My preferred approach is to pick an open source project with a business model similar to your own.
For example Automatic (the parent company of wordpress.com) has about 400 employees and they serve 131 million customers per month. Compare this to Facebook who serves 129 million customers and has around 10,000 employees.
If Wordpress was closed source, they would not be anywhere near as successful as they are.
Another good comparison is Windows compared to OS X and Android. Microsoft was extremely successful when their only competitor was closed source (Mac OS).
But once Apple started using open source for the core of their software platform, and later on Google came in and did the same thing with Android, those those two companies managed to create operating systems far more successful than Windows will ever be.
Same thing with web browsers, IE dominated when all the good browsers were closed source. But faced with open source competition like WebKit and later Blink, suddenly IE is unable to compete and only kept alive because of Microsoft's monopoly position.
If you look at Microsoft's more recent projects, they're using open source all over the place. Microsoft has recognised that being closed source is a massive disadvantage.
Try to find a similar comparison to those ones, where open source allows a project to be far more successful than if it was closed source in a way that matches your own project and explain why open source will work for your project.
As for getting them involved, about the only thing you can do is encourage them to be active in your issue tracker.