An administrator creates user accounts and sends invitations to set up their ArcGIS Online for Organizations accounts based on particular usernames. After 14 days, the user accounts that have not been activated are removed from the Pending Members list in the organization. For an account that is not activated, the same username cannot be reused, activated or re-created by the administrator.
This issue occurs because the invitations are valid for only 14 days and expire after this time.
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