Website: Benchmarks can be found under Start > Dashboard. Filter on a group and click on "See benchmark" next to the question results. 
Mobile app: Open your app and go to Insights, click on the button “See benchmarks” in the top right corner of the question. 
If your organization is using standard questions from &frankly, you’ll be able to see external benchmarks. If you are only using tailor made questions, you’ll be able to see internal benchmarks if the survey was sent to at least 40% of the users in your account.