If you receive this message it’s usually because a user in your organization is deleted, and re-created using the same domain and same account name. This may have been because of a user was deleted by accident, or a previous employee who left your organization and was then rehired.
For example, my account in the “NA” domain with the same account name of john.smith (NA\john.smith). The account was deleted and re-created with the same account name (NA\john.smith).
Because SharePoint stores SID information with the user in it’s tables, the account will no longer work until you either
1) Remove the user completely and re-add, or
2) Use the STSADM -o migrateuser command
You can try removing the user from the Site Settings, Site Administration, Manage Users section but it they are not listed you need to use this migrate user command from the BIN directory of your SharePoint server:
STSADM -o migrateuser -oldlogin na\john.smith -newlogin na\john.smith -ignoresidhistory
You should get an “Operation completed successfully” message and then be able to add them to your Site.