I invited three friends (not at the same time but all within a week) to join duolingo who all accepted my invitation and who all started using duolingo. I invited them because duolingo promises to give you one month of free use of the add free version. Still, I only got one free week of duolingo plus and so far no response from duolingo to my complaint. User's recommendation: Don't invite your friends thinking it will get you a month of free use of the plus version.