The Interactive group holds all users who logon to a machine interactively (after pressing Ctrl+Alt+Del).

You can add a built-in group to a local group, but not from the Windows GUI (using NT's User Manager - MMC). Instead, you must use the Net Localgroup command. The following command adds the Interactive built-in group to the Backup Operators local group:

Net localgroup "Backup Operators" Interactive /add

In the Windows GUI (for example, in the properties for the Back-up Operators group), the Interactive group will appear as NT Authority\Interactive.