We have to support a few networks that have their own Active Directory/File Server. This same company will have Sharepoint and Office Communication Server hosted (for example). Is there any simple way to have the local AD login to the hosted server for users?
SnapOverflow
The simple way is to establish a trust with the hosting provider so SharePoint can just use the out of the box stuff to authenticate / authorize the users.
If that is not an option then you can move to the next step which would involve setting up Forms Based Authentication and an ASP.Net membership provider to your AD via LDAP.