You add a user by selecting the "Manage Users" button on the opening screen. If you are on a different screen, you can navigate back to the User screen by tapping on the "Tap" icon on the far left of the bottom tab bar. If the Tap screen is already showing an exercise, then tap the "User" navigation item on the top left navigation bar.
After tapping the "Manage Users" button, tap "Add New User" and type in a user name. To save the user name, tap the "Done" button. No other information is required for the user.