Allow creation of user and invitations via API

[mharveyDC]

Is your feature request related to a problem? Please describe.
Being able to create a user via the API, and generate an invitation would be great to allow for automating the creation of a user, generating an invitation for the user, and sending them an email to get their account setup.

Describe the solution you'd like
The ability to create users and invitations via the API
Alternatively, allowing for the creation of a user, creating an invitation, and sending it to the new user, and using a notification rule to inform a group of the creation of the user would be amazing.

Describe alternatives you've considered
Manually creating the user, assigning groups, and emailing the user with the invitation.

Additional context
Allowing this would be good for automating the entire creation of a user, from the user itself, to creating an invitation, and sending it to the user, and notifying a target group.