To add a new system user/administrator to you account, just navigate to the 'System users' section of the Admin and look for the blue 'New' button.
What If I can't find the button or it's greyed out?
There could be a couple of reasons for this.
A. It's most likely you don't have the permissions to create a new user. You have to be have 'super-powers' within the account to be able to do this. It's up to the user who created you to give you super-powers or not and you'll need to speak to a user that has super-powers to enable this for you.
Another reason could be that you've reached or exceeded the limits of system users on your account. You can always add more by upgrading your account or adding a Top-up.
Creating the new system user
This part is rather straight forward. Simply enter the new users details and decide whether or net they too receive super-powers. Remember users with super-powers have full administrative access over the account and can make changes to settings and the account subscription type.