While the default mechanism for deploying VSTO v3 Add-Ins is ClickOnce, there there’s now also a documented way to deliver VSTO solutions via an MSI installer. By extension, this should mean you can push it out via group policy (or SMS or whatever).
Mary Lee’s done a good post on this: http://blogs.msdn.com/vsto/archive/2008/04/10/deploying-an-office-solution-using-windows-installer-mary-lee.aspx.
The code and steps outlining creating a setup project are available here: http://code.msdn.microsoft.com/VSTO3MSI.
Misha Schneerson has done a 3-part blog post on deploying your VSTO Add-in to all users (using HKLM instead of HKCU):