@nx-vijay how many routers are between your fog server and your client computers? If its more than the one pfsense server you have in your office then you will need to enable igmp routing on all routers in question.
But then I would have to also question if you really need to multicast (send the same image to each computer at the same time)? Or would unicasting work just as well. Remember with multicasting the slowest computer (link) in the multicast group sets the speed for the entire group. So if you have one site behind a 1Mb DSL connection and one at HQ on 10GbE that computer behind the 1Mb will set the imaging speed for everyone.