Also, you might want to go into control panel>administration tools>services and make sure at least one is running the computer browser service.
Even if they're in different workgroups or not registered w/ a master browser they can be accessed by going into "My Computer" and typing \\computer-ip-address. Also, firewalls can block the other computers from registering with the master browser.
IMO, Netbios is clunky and has been since it's inception... I lol at the way Netbios name registration works more smoothly in OS X and SAMBA than Windows.