|That's likely not a domain login. It looks quite similar but most likely it's just a request for valid credentials to access the share. Which is to say, you need to provide the username/password of an account that has been added to the ACL of that share.|
In a workgroup environment, it's always best to have a valid account on the target machine that reflects the user/PC attempting to connect to said share.
You create a shared folder on PC1 that you wish to give Sally and Ken access to. The smart thing to do is create an account on PC1 for both users using exactly the same credentials they use to login to their respective machines. (ie: username/password)
Add their user accounts to the ACL of the share and assign appropriate permissions.
Now when they go to connect to the share, they will not be asked to enter any credentials.
Without the accounts, then you are asked to login with appropriate credentials (ie: a valid username/password that is added to the share already).