LDAP authentication strategy for enterprise web application?

We are building enterprise web application and it will be hosted on a centralized cloud server. More than one organization will be using this application, and for authenticating the users we will be using LDAP authentication. Each organization maintains their own active directory system.

Now, If I want to authenticate the user when he entered his credentials how do I recognize which organization he belongs to so that I can connect to right LDAP server to validate his credentials.

NOTE: There are multiple Active Directories. The number goes up as we get more clients.

Any help?