Having the same workgroup name on all machines in a lan is a Samba myth. I have 3 workgroup names on my home network and everyone can see and share directories just fine. It does make it easier to work with however when you have everybody displayed in one place.
Anyway to answer your question ( finally
Type gksu gedit /etc/samba/smb.conf
Find the following line ( it's somewhere in the very beginning of the file ):
# Change this to the workgroup/NT-domain name your Samba server will part of
workgroup = workgroup
Change it to
Save the file, open a Terminal and type sudo service samba restart