How do you make a new user into an admin?
When you invite a new user, you can choose to check the box "Invite as administrator":
How do you make an existing user into an admin?
If you want to make an existing user into an admin, you click the user icon in the top right corner of the dashboard and then "My Company". Press "Users" to get to the page where you can change the access settings.
To change the access, simply press the "Make into admin" button next to the user's name.