How to find which users are currently logged in?

The easiest method to find who is logged on to your system is the use the who command, a part of the gnu coreutils package. It can be used as an ordinary user with no options or with my own favored option which enhances readability:

andrew@ilium~$ who -H
NAME     LINE         TIME             COMMENT
andrew   tty1         2016-05-06 07:34
andrew@ilium~$ 

At times you may see yourself or other users apparently logged in more than once with the use of pts/0and pts/1 in place rather than tty1, This indicates a 'Pseudo Terminal Slave' (= pts) and simply shows that you or another user have opened an application such as xterm, sshd or similar process that creates a pseudoterminal master and slave pairing.

More information concerning pts in the referenced AskUbuntu thread, and the man page link from within the accepted answer:

• AskUbuntu: What does “pts/” in the output of w mean?
• pts(4) - Linux man page

All of these answers are excessively bloated. Three characters for a command? Please.

The fastest way to find who's logged on and doing what is:

$ w

You can use the command users to see who is currently logged in. Take care.