You can use the who command to find out who is logged in, where they are logged in, and when they logged in. To use who, simply type who and press <Enter>:
$The tty number that follows each person's login name tells which terminal they logged in on. If people are logged in on several terminals at once, they appear once per login in the who listing.
whobackup tty01 Jun 28 07:56 perry tty002 Jun 28 09:14 theresma tty003 Jun 28 08:49 joseph tty004 Jun 28 11:20 liz tty005 Jun 28 09:02 cecile tty006 Jun 28 10:06 kate tty007 Jun 28 09:35 liane tty009 Jun 28 09:41 bridget tty011 Jun 28 08:06 sarah tty001 Jun 28 08:06 nigel tty008 Jun 28 08:02 gudrun tty010 Jun 28 08:09 susannah tty013 Jun 28 09:14
You can find out when someone logged in by searching for their login name using grep:
who | grep sarahsarah tty001 Jun 28 08:06