How did somebody I don't really know, put a file in my shared folder?

@Mark

Thanks for the answer. Not to say that you are wrong, but wouldn't it have said join the shared folder at some point? It's not like I clicked a link in an email without reading it. I opened dropbox to do something else & the home screen said I had something in my shared folder. Confused I clicked on the shared folder to see what was going on, &  it showed a PDF in the shared folder. It wasn't until I clicked the 3 dots that I saw the file was from the the kickstarter creator.  To be clear it said I needed permission to open the file (which I didn't ask for). Wouldn't dropbox have used the word join or share at some point, if I was asking to access someone else's shared folder or the folder have had their name on it?

I think you are right & they just used the email they got from Kickstarter to automatically send the info out. But, thinking about it as closely as I can, I can't see any point at which dropbox gave a hint that I was either asking to or agreeing to join a shared folder. I was paying attention when this happened. Even looking at it now, I don't see any sign that it isn't mine, other than I don't have the option to delete it.

Assuming you are right, how do I exit the group? I'm not seeing anything in my subscriptions or the shared folder that indicates a way out.