Even though this Web Part is heavily promoted and is supposed to enhance/replace the good old CQWP, it doesn't work right away as you'd expect (at least in SharePoint Online).
Let's imagine you create a blank page and you add the Web Part.
You then configure your query - it'll be very simple "@*onmicrosoft.com" so that I can get all the people from my demo tenant
Some people suggested that this Web Part is not available in SharePoint Online Plan 1 and they've been in touch with Microsoft Support .... which I have to use in production... but I couldn't find an official source stating that. I think that's old information, I think the web part was not even available in SharePoint Online in the beginning. I know it's definitely not available on-premise if you don't have the Enterprise version.
I've also tested this on my demo tenant which is on SharePoint Online Plan 2... and I got the same results. Turns out pretty simple, I just had to enable the Cross Site Publishing feature on the site collection and the web part works.
+ a little bit of CSS = a sample of my desired outcome:
But that will be another blog post :)