I had the same issue when trying to copy folders from one HomeGroup computer to another. When using a PC that had joined the HG, I could pull folders from the PC that had originated the HomeGroup setup, but I couldn't push folders back to it. I would get the Destination Folder Access Denied error.
Sharing and privileges for appropriate local users on both PCs were OK (Full), but I found that on the Security tab of the Properties of the intended destination library Documents folder (on the HG-originating PC), there was a User/Group called HomeUsers on this PC, that needed to have permissions set to Full. Copy to that folder from the satellite PC worked after that.