I think of participatory design as *bringing customers or users into the design process*. So, instead of just talking to people and observing them as we do with research, we also bring folks in for parts of the design phase to help figure out solutions. If you look at a lot of generative research as learning about user problems, you can think of a lot of participatory design techniques as working with users to generate the solutions to those problems. For example, one of my favorite participatory design techniques is the *card sort*.
Let's imagine that you're creating a system for job seekers to browse through various job openings. You've got enough different types of job descriptions that you want to split them up into categories. But which categories? Which jobs go together? Do you want to sort them by type of job? By location? By salary? By all of those? By something else? More importantly, what would your *users* think was the most intuitive way to sort them? One option is to create cards – you can make them digitally with certain types of card sorting software, or you can use actual physical index cards if you're in the same room.
On each card, you might write one of the actual job titles, maybe include some other information about the job that somebody would see when browsing. You'll then want enough to be representative, but not so many that it would take the user hours and hours. The actual number that you need can depend on the sort of taxonomy that you're creating. So, instead of giving you a number, I'll just tell you to practice yourself before trying it on another person. But if it's more than 50, it might take a really long time; but if it's fewer than 20, you probably won't get good info. When you give the cards to a participant, ask them to sort them however they think makes the most sense.
If you're doing it in the same room or over some sort of screen share, have them think out loud. Have them do it in small groups and talk about it with each other so that you can hear about *how they think of the different groups*. Of course, as with any user-involved activity, you'll do this with multiple participants, hopefully all of whom have different perspectives so that you can see *where the commonalities are* and *how different people think about finding jobs*. But it's a super-useful, very inclusive kind of research.
The important thing to see here is how you've *included your users*, though, in the actual *design process*, not just research. Instead of thinking really hard about how *you* would separate jobs into categories, you've gotten direct user feedback about *how they do that*, and that's important because they're the ones who are going to be looking for jobs, not you ... hopefully.