- Open Users & Groups preferences in System Preferences.
- Click on the lock icon and enter your admin password as requrested.
- Click on the Add [-] button under the users list.
- Configure the new account for your wife adding her username and password.
Note that you can give her admin status like yourself or you can change her account to a Standard or Managed account as you wish.
I think Kappy meant, in step 3, Add [+] button.
John Detroye, an Apple Education Consulting Engineer, said that deciding who should have "Admin" access was a difficult decison in general.
He says his rule is that anyone granted "Admin" Access should be capable of undoing any mess they make while using that special Access.
Apple_IDs are more global in nature, and are used when dealing with Apple Web sites, iTunes, iCloud and stuff like that.
What it is asking there is much more mundane:
"What do you want to type as your name to log in to this one Mac."
This name will also be used to create a sub-directory under /Users, and that is where all her files will be stored. So it must meet a few filesystem rules, such as it cannot contain a colon, or start with a period.
Account Name: this is the desired username. Should not contain any special characters other than the "_" underscore. A good username might be something like, firstname_lastname. This is case sensitive, so use all lower-case.
Password: this is the desired account password. Should be a random mix of upper/lower-case letters and numbers. A minimum of 11 characters is recommended. Do not use names, words, birthdays, etc.