There's a ton of things every user should know about Linux / Unix, and I'll be the first to admit that I don't know nearly as much as I should. Here's some basic stuff though (I'll enumerate it as much to remind myself as to inform others) :
user[add|mod|del] - Administrate users. As with anything else, read the man pages and learn the arguments they take inside and out.
group[add|mod|del] - ...same thing, just for groups.
chsh - Change your shell
echo $0 - Get the name of the shell you're currently running
whoami - Lets you know what user you're running under
users - Lists all the users currently logged into the system.
echo "stuff" /dev/pts/[some integer here] - Echoes text to the running pts, good for command line communication with other users.
Some important locations:
/etc/groups - Stores all of the system's user groups
/etc/passwd - Stores all of the system's user information
There's a ton more that should go in here, but I need to finish this post and start a new one. So yeah ... done.