How can I make shutdown not require admin password?

Richard Holloway's answer is not actually the way PolickKit authorisations are meant to be granted. The files installed under /usr/share/polkit-1/actions are not meant to be modified. Instead, you should modify the authorities under /etc/polkit-1/localauthority/50-local.d/.

Here's how you do it for this question:

Create a file named /etc/polkit-1/localauthority/50-local.d/allow_all_users_to_shutdown.pkla and edit it using sudoedit to look like this:

[Allow all users to shutdown]
Identity=unix-user:*
Action=org.freedesktop.consolekit.system.stop-multiple-users
ResultInactive=no
ResultActive=yes

Then create another .pkla file in the same directory. Use any name you like ending with .pkla, for example, allow_all_users_to_restart.pkla, and fill it with these contents:

[Allow all users to restart]
Identity=unix-user:*
Action=org.freedesktop.consolekit.system.restart-multiple-users
ResultInactive=no
ResultActive=yes

References:

• polkit Reference Manual: pklocalauthority
• ArchWiki page on PolicyKit

You do not need a workaround, just change the policy to allow you to shut down without authenticating as admin for shutdown and reboot when multiple users are logged in.

Edit the file /usr/share/polkit-1/actions/org.freedesktop.consolekit.policy using your favorite text editor. You will need root permissions.

Change the section relating to shutdown when others are logged in from

  <action id="org.freedesktop.consolekit.system.stop-multiple-users">
    <description>Stop the system when multiple users are logged in</description>
    <message>System policy prevents stopping the system when other users are logged in</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>auth_admin_keep</allow_active>
    </defaults>
  </action>

to

  <action id="org.freedesktop.consolekit.system.stop-multiple-users">
    <description>Stop the system when multiple users are logged in</description>
    <message>System policy prevents stopping the system when other users are logged in</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>yes</allow_active>
    </defaults>
  </action>

and the section relating to rebooting when others are logged in from

  <action id="org.freedesktop.consolekit.system.restart-multiple-users">
    <description>Restart the system when multiple users are logged in</description>
    <message>System policy prevents restarting the system when other users are logged in</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>auth_admin_keep</allow_active>
    </defaults>
  </action>

to

  <action id="org.freedesktop.consolekit.system.restart-multiple-users">
    <description>Restart the system when multiple users are logged in</description>
    <message>System policy prevents restarting the system when other users are logged in</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>yes</allow_active>
    </defaults>
  </action>

And that will allow you shutdown and reboot the PC when multiple users are logged in. Whether you want to do that is a different question.

There is a better way. If you have dbus-send installed, you can shutdown via dbus without the need to escalate to root privileges.

I can't remember the page where the documentation is, but one Archlinux user figured this out.

Shutdown:

dbus-send --system --print-reply --dest=org.freedesktop.Hal \
          /org/freedesktop/Hal/devices/computer \
          org.freedesktop.Hal.Device.SystemPowerManagement.Shutdown

Reboot:

dbus-send --system --print-reply --dest=org.freedesktop.Hal \
          /org/freedesktop/Hal/devices/computer \
          org.freedesktop.Hal.Device.SystemPowerManagement.Reboot

Suspend:

dbus-send --system --print-reply --dest=org.freedesktop.Hal \
          /org/freedesktop/Hal/devices/computer \
          org.freedesktop.Hal.Device.SystemPowerManagement.Suspend int32:1

Hibernate:

dbus-send --system --print-reply --dest=org.freedesktop.Hal \
          /org/freedesktop/Hal/devices/computer \
          org.freedesktop.Hal.Device.SystemPowerManagement.Hibernate

Regards.