I know that the PowerShell command to set a single user's Skype for Business PIN is this:
Set-CsClientPin -Identity "User Name" -Pin 123456
This page seems to help:
https://technet.microsoft.com/en-us/library/gg398929.aspx
However, I can't figure out how to set the PIN for all users with “Audio_Video_Dialin_Conferencing”
to 123456