The easiest way is to use split approach.
But to achieve this, you need to add a username inside the directory you want to share:
# mkdir /sharedir
# ln -s /share_dir /sharedir/username
# ln -s /sharedir/username /usr/share/cagefs-skeleton/share_dir
# echo "%/sharedir" >> /etc/cagefs/cagefs.mp
# cagefsctl --remount-all
Please note that first you create empty ‘sharedir’ directory, and the actual content is located in ‘share_dir’