How to Manage Signatures using Group Policy

Signatures are stored in the roaming section of the windows profile, under %appdata%\microsoft\signatures. You can push signature updates into these folders using a Windows Scripting Host (WSH) script.

You can create signatures using AD data, either using a script or a utility.

Signature assignments are in the registry, in the account settings within the profile key. If you use the default profile name, the key for the first account in the profile is (assuming you use the default of Outlook as the profile name) Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\xx.0\Outlook\Profiles\Outlook\9375CFF0413111d3B88A00104B2A6676\000000nn (where xx is your version of Outlook and nn varies with each account in the profile).
The key cannot be set using group policy templates, although it could be set using a logon script as long as all users using the same name for the profile and the account's key is at 00000001.

For older versions of Outlook where the default signature is used for all accounts, the key is at at HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Common\MailSettings\ (where xx = your version of Office). This key is in the Outlook .adm file under Outlook, Tools, Options, Mail Format, Signature.

Alternately, you can add disclaimers to all messages using Exchange transport or mail flow rules or using utilities installed on the Exchange or SMTP server. Cloud-based signature services are available for Office 365.