Just Dave,
unfortunately SME server doesn't support (by default) domain authentication. It meant to be the primary (or the only) server on the network that does user management by itself.
There are some how-tos on this site that help you to add domain authentication to SME, but if you go this route SME will become just another samba fileserver on your network and not any better then the one you already have. Why do you need the whole cow, then you only looking for glass of milk?