Users can be added via the Users panel in Settings, but you cannot control groups this way.
New groups can be added with
sudo groupadd <group>
sudo usermod -a -G <group> <user>The -a flag is essential to append groups to the user, otherwise their list of groups is replaced with this group. <group> can be a comma separated list of groups.
Adding someone to a group won’t take effect unless they log back in. You can use
newgrp <group>to add the group for the current session
https://askubuntu.com/questions/79565/how-to-add-existing-user-to-an-existing-group