Multicast Routing?

Hopefully someone can help me out, as I'm not well versed in multicast at all.

We're implementing a paging system where the paging server sits in a central location and the endpoints are in a separate building connected by a MPLS network. Here's a brief network sketch. When the server pages the endpoints, it uses multicast. If it's possible for the devices at site B to receive the multicast streams, I'm assuming I have to configure a PIM domain at site A, and configure site B to forward IGMP to the router at site A?

My Site B config would look like this?

vlan 20

ip igmp

igmp-proxy-domain "SiteA" 172.1.100.1

I'm not sure at all what I need to do at Site A, as I'm not clear on the differences between PIM-SM/DM etc. Any help would be appreciated. Thanks!