I have a Win 7 laptop from work on my company's domain. I can access shares on my home network's work group fairly easily. I cannot access some services, like TFS, when a login dialog pops up. It only allows me to login in from my company domain. Another option is not given.
Simply type the name of the Workgroup in front of a valid username on the destination PC. For example - if the PC hosting the files you want to access is called PCONE and you know a valid login on that machines local users group is username and password. You would enter the credentials as so in the pop-up:
The pcone\ tells the credentials window to ignore the domain it auto-populated and to validate against the pcone user cache instead.