In Leopard, Directory Utility seems to have replaced Dir. Access in tiger. Unfortunately, now there is no way to change the workgroup that the Mac is part of. Google has said that I should click "SMB" in dir. access, but there is no "SMB" in Dir. Util. Under the smb setting is where you change the workgroup. Does anyone know how to fix this?
Thanks in advance.

Not sure, sorry. You could check /etc/smb.conf , IIRC there's a workgroup setting there, though it may only be for running a server. There may be a way to configure workgroup in System Preferences.app.

I've never used 10.5, sorry.