Someone can correct me on this, but I don't think there is a specific Windows permission. You might be referring to the ability to create a SQL Server login using Windows authentication. If that is the case, it follows the SQL Server permissions. When creating the login, you must specify the rights to that login to create other Database logins. Per BOL:
Requires ALTER ANY LOGIN permission on the server. If the CREDENTIAL option is used, also requires ALTER ANY CREDENTIAL permission on the server.
I hope this helps...
Author - Professional SQL Server 2005 XML