One login for 3 diff directories

Hi …

Firstly just a thanks to Josh and a couple other members for feedback on previous issues.

I have a another tricky hurdle .
I would like to know how to go about having one suer name password for 4 installs.

It looks like this
http://www.mydomain.com ( main site )
http://www.mydomain.com/charter ( directory listing for charter yachts )
http://www.mydomain.com/yachtingbusiness ( directory for businesses )
http://www.mydomain.com/jobs ( recruitment site )

How can i get this to work … signup/login on the main site ( with forums, etc ) and then be logged in and able to manage listings etc on all 3 of the other subdirectory sites / directories?

Thanks for any tips
Tyrone