Facebook and iPhone developer required for facebook sign up

We have launched an iPhone application and a website. The website uses xenforo forum accounts as our user accounts, so when someone signs up online it creates a xenforo users account, and also when someone signs up using the app it creates a xenforo account. Users that sign up with either the app or the website can use the log in details for both the app and website, it is one backend. What we want to do is integrate facebook in our app, so that users can sign up with Facebook to avoi filling in the forms. What needs to happen is when someone signs up with Facebook in the app, it needs to create the xenforo user account, so that the users can then log in in both website and app with facebook details. If you require any more information please pm.