User email has to be unique across the Aurora app.
Please note that in contrast to the Invite User endpoint this will not send the user an email to join the user. The created user account will hence not be able to set their password.
Creating a user without inviting them is a required step if you are using SSO through OpenID connect to authenticate the users before accessing Aurora's application. This will allow your users to use Aurora without having to type in their username and password before doing so, but require you to run an OpenID Connect server (proving the Identity Provider functionality to Aurora). See OpenID Connect specification for more detail.
Please note that certain identity providers such as Salesforce, represent their user and organization IDs differently in the UI compared to what is sent in actual API requests. If you are using Salesforce as the identity provider with the OpenID Connect standard, please specify the long 18 character user and organization ids when creating a user. See here for instructions on how to convert a 15 character ID to an 18 character one.