Invite (or provision) a single user
POST/users/invite
Creates a user in the org, or an invitation when sendInvite is true (the default). If the email already belongs to a user in the org, that user is returned with status: linked (or updated when projectIds added memberships); the role is never changed. Supply externalRef for idempotent upserts: a second call with the same reference returns the same user or pending invitation.
Permission: write:user.
Key scope: organisation keys only. A project-scoped key gets 403 scope_violation.
Add-on: requires the Illumera add-on. Without it the call returns 403 with reason: addon_required.
Request
An existing user or invitation was matched.