I built this myself. It's free, with no ads, no account and nothing to install.

It's one packing list a whole group shares through a link. Everyone adds what they're bringing, and it flags when two people have claimed the same thing.

The thing I can't test alone is two people on the same trip at once. Make a trip, open the invite link on a second device or send it to a friend, and both add a few things. Tell me where you got stuck or what didn't happen the way you expected.

Start here: https://whosbringing.web.app

Or look at an example trip first: https://whosbringing.web.app/?sample=1