facebook API limitations (iframe)

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 开发者_C百科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.