BuddyPress.org

I would need this as well at some point

Say you have a network of four sites. When a user tries to log in one of the sites (‘New Community’) with a username and password from one of the other three sites, he gets this:

ERROR: Invalid username. Lost your password?

Would it be very complicated to write a plugin that checks if the entered username exists in the other three databases? And then verify the user’s password by checking in that same database? And then enter that user’s data, including username + pass, into the ‘New Community’ database?

Could it copy some functions/code from the new Facebook Login plugin?