Apologies if this isn't the best site, I've search for an answer but can't find anything quite right. I don't actually now the correct terminology I should be using here, so any pointers will be appreciated.

I have a web application that accessed by many different users across different organisations. Access is provided by each user having a unique username/password which is stored in SQL (database fields are customerID, userID, username).

Some organisations are now asking if we can change this to allow "Active Directory single sign-on" so that users don't need to remember yet another set of login details.
From research I can see how this is achieved using OpenAuth and Google (etc), but I know hardly anything about AD and can't find much information on this (again I'm sure it helps when you know the terminology).

Is this request even possible to achieve, given that most users will be from different (and unrelated) organisations? I saw on a Microsoft Build video not long ago that there is some kind of replication service for AD to allow Cloud authentication. Is this what I should be aiming for?