Before you start adding the category to a lot of profiles, I'd point out that "Descendants of Hans Herr" is part of a bigger project:
I'd first contact the people in charge of this project, and talk about what they're trying to do, and which profiles are supposed to have the category.
Otherwise, it looks like you're already connected to Herr-99, through your family tree. Once you're sure you should add the category to the profile of some of your ancestors, you can simply just paste the following (double square brackets and all):
[[Category: Descendants of Hans Herr]]
onto the top of the profile biography.