groupadd twousers
usermod -a -G twousers user1
usermod -a -G twousers user2

chgrp twousers /home/user1/folder
chmod g+rwx /home/user1/folder

# cd /
# mkdir data

# cd data
# mkdir shared_folder

# cp -p  /path/to/folder/* /data/shared_folder/

# group add shared
# usermod -aG share bob
# usermod -aG share alice

# chgrp -R shared /data

# chmod -R g+rwX /data