CompleteFTP's web-based FileManager is able to share files in the manner you're describing. If you need a user interface exactly like you're describing then you can develop it with CompleteFTP (see here). Our server-side scripting language is JSS, which is Javascript + an extensive API for integrating with CompleteFTP and performing common operations. Although it's not currently documented, the API does allow integration with CompleteFTP's file-sharing system, allowing you to manage user shares, including creating new ones. We'd be happy to advise you on how to get started on your development project.