In this post, I will introduce how to setup an sftp server by using ssh

su -
groupadd sftp_users
usermod -G sftp_users my_username

Then:

$ emacs /etc/ssh/sshd_config

# Search the line below and replace it
Subsystem sftp /usr/lib/openssh/sftp-server
# by this one
Subsystem sftp internal-sftp

# Now each time that a user connects from the sftp group, 
# we are gonna apply theses rules:

Match Group sftp_users
 X11Forwarding no
 AllowTcpForwarding no
 ChrootDirectory /home
 ForceCommand internal-sftp

Finally:

systemctl restart ssh

StackOverflow - SFTP difference between internal sftp and sftp server

Edit: If you need to remove a user from a group:

gpasswd -d user group

or debian includes a tool:

deluser user group