Well by default users automatically use their Windows IDs to open documents from the AccessPoint. But it sounds like you've moved away from the default behavior. If you're talking about Section Access, then you can use NTNAME to check a user's Window's ID. If you changed the QVS to use DMS authentication instead of NTFS, then you have 1 of 2 possibilities: (1) you are using a login.htm page for user login: if that's the case, you're forcing 2 logins and you can't combine it of course; or (2) you are using some external DSP in addition to the Windows AD: if that's the case, you can add the user's Windows ID to the distribution as usual.
Hope this helps.