When you have a SharePoint list with a people picker column you can enter any person from your Active Directory and add it to an item. When you open the list in datasheet view the people picker column is pre-populated with the data from the hidden all users list. You can find this list at: http://yourdomain/_catalogs/users/simple.aspx.
When you want to add an account that is not in that list yet you will get an error. The error says that the item is not found in the list although the user can be in Active Directory.
Is there a workaround for a power user without access to the central admin application? Yes! Of course!
- Create a custom list with 1 people column and make it multi selectable.
- Add an item to the list and enter the name of the account in the people field.
- Save the item (you can even delete the item now or delete the entire list)
By saving the item a new entry is created in the hidden all users list and now your person will be in the dropdown list when you are in datasheet mode.