You can add additional admins by first adding a user and then assigning them the Additional Admin role.
1. Add your additional user. You can check here on how to add additional users to your account.
2. Once the user is added go to User Roles settings and assign them the additional admin role as shown below.
3. Search the user(s) you wish to give additional admin role to and click on the add button. Make sure to click on the Save button to save your settings.