You can configure user credentials verification across domains. You can configure user logon to locations other than the default. By default, Windows attempts to log a user on to the domain the system is attached to, or to the local system if it is not part of a domain. If the user does not exist in the default location, you can configure the system to try:
With this option, the process searches for a corresponding user ID in the local system and the domain forest, and tries the logon in the first domain where the user ID is located.