$ sudo mount -t cifs -o username=your_osx_user_name,uid=$(id -u),gid=$(id -g),forceuid,forcegid, //server-address/folder /mount/path/on/ubuntu
Credit goes to https://unix.stackexchange.com/questions/68079/mount-cifs-network-drive-write-permissions-and-chown.
After you share a folder on your mac, you can run this command from a terminal on your ubuntu box.
Make sure to run apt-get install cifs-utils first. The UID and GID parameters set permissions on the folder in ubuntu so you can access the files.
I did this because I wanted to process image files on my mac with Imagemagick, but it would not install with homebrew, presumably because I need to upgrade OS X.