The basic setup would be as follows:
- Backend: Database (MySQL), Web server (Apache), with server side scripting (PHP).
- Client: iOS device with developed app.
- Communication: use HTTP client/server model, communicating with something like JSON.
This is much the same setup as a web server, but instead of serving html/css/javascript etc the results will be JSON.
As far as implementing specifics such as login in, and sharing data between users, this is purely dependent on your implementation. This is not trivial, and not something that can be easily stated in a single post.
Hope this helps.