This article explains how to add people's profiles to your Amazon Fire tablet. You can do this through Amazon's website, and may require a free account.
Amazon Fire Tablet: How do I add a profile?
Amazon Households currently allows a total of six people on an account. Of those six, two can be designated as adults, while the other four can be children or teens. Adults and teens require an Amazon account, and children can be added directly by their parents.
Go to Amazon Household and select the appropriate name. Adults and teens will receive an email to confirm they want to join the household, while children can be added by their parents.
Once they’ve confirmed their accounts, swipe down from the top of your tablet and tap their profile picture in the top right of the menu. This will open a list of all the profiles in your household. You may need to restart your Fire tablet before you see a proper list.