How can my friend add files to that folder that I have shared with him?

Now that I've shared a folder and given my friend the permission to "Edit" it, how can my friend add files to that folder that I have shared with him? What I want is an area where we can share files between us - we're not a business or a "team", we're just friends who want somewhere secure where we can share files in one place. Does Dropbox Plus allow that? If not - why not, because why else would you share a Dropbox folder and give someone else the rights to edit it???