Imagine you are working for an software (I.T.) company and you have a new employee who needs to use the software. At the same time, some existing (software) users need some additional training so they can get 'up to speed' on some new features of the software.
Your boss asks you how the training of the new user/employee is going. You would be able to say "Der Nutzer lernt in der Gruppe". I.e. The user is learning in the group (that you have set up.)
Thank you, Robbie_Canada. That helps. I still think it's a poor sentence, Duolingo, since without a context, it is unclear. Perhaps putting an adjective, such as "computer," in front of "user" would help give the sentence meaning. "User" in American English many times conjures up "drug user," which I hope would not work in THIS sentence!