On sign-up (i.e. when accessing the app), we need to get user's permission to invite their friends to use the app. When a user has finished using the app and the picture displays on their wall, we need to share the post with all of their friends automatically
Can this be done in FB application running from iframe? What would be the easiest approach to do this.
You can't automatically send out invites. You can present the user with a form to pick friends to invite, but your app can't do it on their behalf.