Every account needs a first name, last name, and email address.  Company Name is optional and Link Code is unneccessary.

Make sure they are a "User" and toggled to "Active", and then "Create".

If the email address is already in use, the user creation will fail.

There are several ways to find a user's account; search by their email address, name, company name, or even just a combination of any information.  This can come in handy when trying to locate an account when the user swears they have licenses but nothing is tied to the email address.  Search the name or studio email extension next.