This article explains how to add additional administrator accounts or promote existing users in macOS 10.15 (Catalina). However, in older versions, the procedure is almost identical.
Mac Basics: User Accounts
An administrator account has the same basic capabilities as a standard user account, including its own home folder, desktop, wallpapers, preferences, music, bookmarks, messaging accounts, address book/contacts, and other account features. What sets an administrator account apart is its elevated privilege levels. Administrators can change system preferences that affect how your Mac works and feels, install software, and perform many special tasks that standard user accounts can't perform.
You only need one administrator account for your Mac computer, but it's easy to give one or two other trusted people administrator rights.
You must be logged in as an administrator to create or edit user accounts. You created an administrator account when you first set up your Mac. Then:
