First you need to share files and folders using these instructions
http://www.7tutorials.com/how-change-workgroup-ubuntu-linux-work-windows
Now you need to set the Workgroup to the same as the other computers on your network.
This may not need changing if your workgroup is set to WORKGROUP
By default this is the automatic value
To change Open the Terminal Window
then type
sudo gedit /etc/samba/smb.conf
check the line workgroup=workgroupname
alter as required then restart your computer