since this is not a domain, you will need to set up local accounts for the users that you want to be able to share the folder. then, in the sharing & security->sharing tab, go to the permissions button. add the local users there. i find it easiest to click the Advanced button, and then select the Object Types button. select only users (unselect the other options). Then click Find Now and add the users.
when the users go to access the share, they will need to use the username & password of the account you set up for them.