Its quite easy to manage users with the cmd tools and here are some examples:

- Create user:
adduser user1

- Create a group:
groupadd group1

- Add user to group:
usermod -a -G ftp user1 (added to group ftp)

- Change password or force a bad password:
passwd user1 (logged in as root changing user1 pw)
passwd (type it as logged in as the user)