Accounts with a subscription including 1000 keywords or above, will have the ability to add an unlimited amount of users to their account. When adding a user there's the option to make them an "Admin".
What is the difference between an Admin and Normal user?
Admin users have access to the account settings whereas "Normal" users do not, that is the only difference.
How to change a user from "Standard" to "Admin" (and vice versa)
Go to Account and select "Users". Edit the user in question, then enable (or disable) them as admin.