[phubeone]

My suggestion would be to create a domain for workgroup A and keep workgroup B the way it is. You can easily restrict workgroup B from the domain.

The other option is to restrict the shares on the server on workgroup A.

I would go with the domain creation, everything is managed centrally on the server and you can easily manage domain policies.

Give us a little more info, what exactly are you trying to restrict?

 

[phubeone]

No what you can do is also right click on the shared folder and remove everyone from the list and add the users in workgroup A to the list and give them the access you want them to have. I would also add the users in workgroup B and deny them any access to that share. I hope that makes sense.