I'm also having problems with this.
I am sharing the drives and not just folders and I have access enabled for everyone. My two computers connected to the homegroup can see/access/read/write to the shared drives but not the tablet.
I have ES installed on the android tablet, I can see the IP address of the computers, I can see the shared folder list, but when I go to open it, it says I have no access.
My computers have no password (should probably fix that) and ES doesn't ask me for any username.
Something worth mentioning is that you can us ES to setup an FTP server so you can access your phone/tablet from your pc. It's painfully slow to transfer files that way though