"Invite a friend" achievement
So there's the silly "Invite a friend" achievement thingies. It doesn't tell you how it wants you to do this.
I know there's no way for it to know that there are several people on Duolingo because I talked them into it, but I've actually entered email addresses into the computer interface of Duo, and not gotten the achievement.
While I've noticed that the phone app doesn't notice an achievement accomplished on computer until I do another module on the phone app, this one never made it through - and there seems to be no way to invite a friend via the phone app. Has anyone managed to "accomplish" this achievement?
(Side note to Duo programmers: It would be fun if, when new people register, they could ID a user as the reason they're joining.)
I had the same problem and reported it to Duolingo as a technical support ticket. It is a rather amusing failure of design process for Duolingo to create an App achievement for "inviting a friend" when the App contains no capability for inviting friends. I invited a friend from the Web interface after the achievement was created, and he joined (!), but I didn't get the achievement recorded.
I would suggest, however, moving this post to the "Troubleshooting" forum, where it might catch the eye of someone at Duolingo able to do something about it, as well as opening a technical support ticket (mine has thus far gone unanswered).
In the meantime, I have resigned myself to life with a Duolingo achievement forever left unattained. Don't worry....the disappointment fades with time. :-)