• To know which users exist:
  cat /etc/passwd | grep -i home
• Delete a user:
  userdel [username]
• Add a user:
  useradd -m [username] (the "-m" flag creates the home directory)
• Set the user's password:
  passwd [username]
• Assign a user to a group:
  usermod -a -G [groupname] [username]
  (make the user re-login to activate the changes)
  (for a user to be able to "su" to another one, it has to be in the group "wheel")
• View to which groups a user was assigned:
  groups [username]
• Remove a group from a user:
  usermod -G [all_groups_the_user_is_supposed_to_have-comma_separated] [username]
  Here I practically reassign to the user only the groups listed and the rest gets deleted.
  (make the user re-login to activate the changes)
• Change the owner of a file:
  chown [newowner] [filename]
• Change the assigned group of a file:
  chgrp [newgroup] [filename]
• Create a group:
  groupadd [newgroup]
  Now you can assign files and users to the new group.
• Delete a group:
  groupdel [groupname]