When you have access to more than one account, it is easy to add a user to a specific account as the admin. First you need to log in with the admin user.
Go to the preferences menu and select the "users."
Then in the library, you select the account you would like to create the user for, and you will see the user's menu switch when you jump between the accounts.
Then click on the "+" and add the user, email, name and cell number. You can also set an expiry date if the user fx is a freelancer.
Set the Privileges, and press the "Sent the Credentials" Then, the user will then receive a mail with the login.