For local (non-network) accounts, look at the /etc/passwd file. It contains all of the local users on that system. You can use many different commands such as less, more, cat, etc., to see the contents.
There are a series of commands, depending on what and how much information you want on logged in users. The commands are: who w users finger (if installed)
The command "finger" can tell you how many active shells are used, the users logged in, and where they logged in.
In Linux the chmod command is used to set file permissions.
vi
date
The 'man' or the 'info' command have documentation about utilities and commands in the system.
$
NIS
cd ls
The TAR command is Create Tape ARchives and is used by Linux and Unix. Information and explanations can be found on the linfo and computerhope websites.
KUser
Use the 'newgrp' command.