Hi. There are several already existent solutions of your taks. The software similar to dropbox (even better than dropbox for some reasons) consists of server part and client parts. Server part written in php with mysql, client parts written for mac, windows, android, ios, web client. And yes, there are admin user and regular users, possible to share files and folders, there are a many useful plugins. I implemented this solution into many different projects, highly recommend.
Are you interesting in such decision? Let's discuss