Dropbox – Sharing Folders with Secondary Email Addresses

Without using any kind of API, from where you'll probably able to get the e-mail of your shared folders, you can check the e-mail from the web interface.

Since there is no such thing as contact list here, you can do the following:

1. Login to Dropbox.com
2. Go to Sharing tab
3. Enter any folder and click on Shared folder options button
4. Once you're there click on the Members tab
5. Finally by clicking on the names there you can send them e-mail, and it's using your native client :)

Another way of checking your "list of shared e-mails" is to create a folder, again from the WebUI, then click on the Share a folder and when you start typing the autosuggestion kicks in. I don't know how many mails do you have, but you can test all the letters (they are not too many) and it really depends on how important this is for you, to go through all that hassle.

They can still share folders and files with you through the web interface, and vice versa - you can share folders and files with them, which they can view through the aforementioned webapp. Only difference is they won't have those files stored locally (unless they choose to manually download them).