Apparently, Dropbox from its server is only able to collect web-addresses that are somewhere on the internet, not in a file (linked Outlook Express, Thunderbird, MS Outlook, Incredimail, Eudora, Windows Mail, Windows Live Mail) on your PC.
So consider to open a gmail-account and put your contacts e-mail addresses there also. In the long run, it's easier than copy/paste each time.
The alternative, of course, is to send a mail from Outlook Express to all those people yourself, and just copy the link into it. If you use BCC they won't see each others data.
But it could be that Dropbox doesn't allow access to those files to them if it doesn't know the email address. I don't know enough of Dropbox to tell if that would make the scheme unusable. Worth a try first, from an another Windows account on your own computer.