To allow users to shutdown a FreeBSD system you basically have two options: add the user to the operator group or install and configure sudo to allow usage of the administrative command. I've already covered the latter option in a previous post so I'll only go over the remaining option here.
The steps consist in becoming root and using pw to add the user (in the example bellow username) to the operator group:
% suTo reboot the system login to the user account that belongs to the operator group and run:
# pw groupmod operator -m username
% shutdown -r nowIf you want to power down the system run this instead:
% shutdown -p nowDo take a look at pw(8) and shutdown(8) manual pages for further information.