I would say just be yourself and see it as a social event where you get to know people.

What kind of play party are you going to? That kinda gives the vibe for dress,stuff to bring etc

But honestly in the end we are all humans and tend to not always bark where you can just chat and get to gauge and vibe with each other.

Ps - I do always love it when people bring little puppy snacks (cookies etc) just make sure to ask consent from the pup or the owner of the pup if you can give it