Mcast source *can* use IGMP to join the group (or another group) but this is not necessary & is application/implementation specific.
At the 1st hop router, packets from new sources sending to new groups are either flooded to all routers (PIM dense) & pruned back or encapsulated in a PIM register packet (PIM sparse) & unicasted to the RP for registration.
Once that is done, if no one is interested in the (S,G), then the 1st hop router essentially just drops all traffic from that source until a d/s device asks for it. Once a device asks for it, 1st hop router forwards on appropriate interfaces.
