From
http://docs.info.apple.com/article.html?artnum=107137
To change the workgroup, follow these steps:
1. Open Directory Access (/Applications/Utilities/).
2. Click the padlock to authenticate as an Admin user.
3. Select SMB from the Services list.
4. Click Configure.
5. Select the name of the workgroup from the list, or type the name of the workgroup you wish to create.
6. Click OK.
7. Click Apply.
8. Quit Directory Access.
Note: Changes in Directory Access may take up to 5 minutes to be recognized by other computers on the network. To force a quicker update, stop and restart Windows File Sharing.