It depends on the flavour of Unix you are using.
There may or may not be a graphical interface.
But you should always be able to use command line interface.
For a start, your 1st question, something like
man adduser
man useradd
You will need root access for executing these commands,
but not for reading the manual.
and 2nd:
man who
man finger
man ps
cat /etc/passwd
hope this helps