Once you have Two-Factor Authentication enabled for your account as an Account Owner, you can choose to make it Required for individual Account Users as well. Here's how...
STEP 1 - From your dashboard, hover over the Account tab and choose Settings
STEP 2 - Click Edit next to the name of the Account User for which you want to require Two-Factor Authentication
STEP 3 - Change Optional to Required for Two-Factor Authentication
By default, Account Users will have Two-Factor Authentication set to "Optional." By selecting "Required," the user will be prompted to setup Two-Factor Authentication before they login.