Welcome to the community!
To understand what's in it for customers, you need to analyze how they interact with you now and why they would want to change their behavior to use a community. You are on the right track by looking at why customers would use a community (meaning what benefits THEY would receive for using it, not what HireRight whould receive).
In short, your customers gain a closer relationship with your company. Remember that in a B2B community, members join not for fun, but because they want to build a relationship with the host company. That means that your team and internal subject matter experts must engage early and often in the community for it to succeed. By doing that, the customer gets the relationship they want, even if they are just reading posts and documentation.
Second, to change behavior you need to remove conflicting/competing information channels slowly. Do your customers get all their information from an account rep? If so, that process can be put into the community with the added benefit to your customers that they can ask questions and read solutions to help them avoid issues that others have faced (an especially big benefit to hiring managers).
There's more, but thinking on this track will give you plenty to discuss with your customer advisory group.