Amending my previous post:
The ProviderID for a user in Amazing Charts is a SQL Server IDENTITY. However, whether or not that is used to refer to the user depends on the table doing the referencing. Some tables reference the ProviderName column - which provides the contents of the username list on the logon screen - rather than the ProviderID column. Actually, I think most of them do. Others reference the FirstName column - this is often seen in the LastTouchedBy column of many tables.
All users are kept in a table called ProviderSecurity.
JamesNT