setting user share permissions

I'm sure this has been asked a lot. I did a quick search on the web and in the forum and didn't see anything at the noob level I'm at.
I have a readynas pro. I have created folders on it through the browser config. Now I want to set specific users and permissions.
In windows explorer for example I can rightclick, pick sharing & security and start clicking add to add any number of domain and local users with any number of permissions.

How can I say domain user john, and dave (who isn't in a domain) can both write to SomeFolder and other assorted domain and nondomain users can read it.
If I set default access to disabled I can check a box that says Write enabled users. But when I enter mydomain\john, dave it says removing access for nonexistent member mydomain\john. But he is a valid 'existing' domain member. Also, when I try to connect to the share and enter dave and his password it fails with invalid name or password


Also, how can I check all content for example for total space used in this folder and that?