It is possible to create entries within People based on a site's Users. This is done by exporting the list of users to a .CSV file, and then importing people from that .CSV file. It is important that you only do this once, as repeating this process will add 2nd, 3rd, and more copies of each person into your 'people' content area.
Export Users to CSV
Go to Tools > Import/Export Users and select the green Export Users button. Download the generated .CSV file to your computer.
Import People From CSV
Go to Tools > Import People, and below the 'One Time Use' heading, choose the .CSV file to upload. Hit the Import People button. A person entry will be recorded for each 'public' user, and all of their titles, phone numbers, and photos will be transferred for you.