The normal passdb back-end that Samba uses doesn't replicate well (if at all,) so some form of database that does replicate well is essential to having a multi-site distributed Samba deployment. For a set of detailed instructions (almost a complete walkthrough) go to the Samba site. In their docs they have a Samba guide that is setup "By Example" that will show you exactly how to do what you are asking to do. Make sure you take the time to go through all the information so that you understand what you are doing, so you can know where to look if something goes wrong (God forbid!)
http://www.samba.org/samba/docs/man/Samba-Guide/
I hope this helps. Samba can be real simple, once you know the specifics (just like everything else in the *nix world.)