Then you can search for traits or stats in the search bar at the top. You can set the filter which only shows people that will accept an invitation to your court. The way to do this is by pressing the '.' on the keyboard, this brings up a 'search character'.
But there aren't always the strongest people around who can help you, therefore you have to look outside your own estates and your own court. I take it you understand that you pick the one with the strongest skill for the right job. The first tip is to build a strong council a diplomat to increase your relations faster, or a spy to assassinate your foes more successfully. A single man cannot achieve much, even if you are the Holy Roman Emperor.