The original way to do this is via the rwho command. This
command depends (as does ruptime) on a program called rwhod,
one of a family of programs called daemons. A daemon is a program
that runs continuously, checking for some sort of event to take place. The
job of rwhod is to interrogate all the other systems it can
find every few moments to ask them who is logged in. In large networks this
puts quite a load on the system and so it is often dilinux.ox.ac.ukd.
An alternative found on some systems is rusers. No daemon
is involved this time; other machines on the network are interrogated for
information on their users when you execute this command. The drawback with
this program is that it often interrogates machines (like PCS and Macs)
that can't answer!
Another program found on most Unix machines is finger.
finger
finger @machine-name
finger username
finger username@machine