I get what you are saying.

You have a workgroup share lets say "dino"
You want to connect to "dino" but first want to be prompted for a userid and pass.

Add a user to the local machine say "MLF"
give the local "MLF" account access to the share, Remove all other accounts.

When ever someone wants to connect to "dino" they will have to enter the userID "MLF" and it's password.