(Note: There may be more than one administrator per account, but there can be only one Owner. Also, only account Owners or Administrators can make a user or manager into another Administrator. The process of changing the Owner of an Account is different - see this Support Document to change the Owner of an account.)
Then, click on the 'Edit' button found under the 'Actions' column and on the row of the user you wish to set as an Administrator:
This will bring up the Edit User page, where you can select what access Level the user is authorized for. Make sure to click on 'Save Settings' after making your selection.
See this support article for more information on the various user access levels.
For questions, comments or feedback regarding this topic, please send an email to firstname.lastname@example.org.