You can create an account called "everyone", but it will be an ordinary account. There's no such thing as giving ownership of a file to all users. You can, however, give everybody read/write access to files in a directory using chmod:
This gives everybody read/write permissions to all the files and subdirectories in /some/directory. Note that new files may not be affected.
sudo chmod -R o+rw /some/directory
A better option in the long term is to synchronize the UIDs and GIDs on the two OSes. Frobber has provided directions, although I suspect they're overly complex. I don't have time to verify and debug simpler custom directions right this moment, though. Perhaps Googling will turn something up. You can also check the man page for the usermod command. This sort of thing is sometimes easier with a direct root login, but Ubuntu doesn't set the system up to permit this by default.